There are actually three sources under discussion, and it would pay to discuss each separately. There is "the midrash," by which Rabbi Falk means Midrash Rabbah on Rus, which I believe is Geonic (though includes some material from Amoraim); Midrash Zuta, which is a later midrash on Rus (900-1000, so perhaps late Geonic), and Rashi.
There is a certain tension between two messages in Rus. If you read the third perek of Rus carefully, you will notice that there is a lot of sexual imagery, which either describes what actually happened or the sexual tension. Thus, for example, Rus uncovers Boaz's feet (meraglotav) and sleeps. And Boaz shakes, and notices the woman at his feet. And he tells her lini po halayla. (Midrash Zuta says Boaz was talking to his yetzer.) At the very least, even if nothing actually occurred there on the threshing floor, these actions were quite forward. On the other hand, he praises her for not going after the bachurim -- לְבִלְתִּי-לֶכֶת, אַחֲרֵי הַבַּחוּרִים--אִם-דַּל, וְאִם-עָשִׁיר.
Often, midrash assumes the role of taking a character who is a shade of gray and painting him or her black or white, emphasizing certain of their aspects. And this could either be because midrash likes starker heroes and villains, or in order to make a greater rhetorical point.
Therefore, in the context of her behavior vis a vis the bachurim, the midrash will take the heroine, Ruth, and stress this positive trait of modesty, especially in order to compensate for the events in the goren. There is also tension in the apparent breach of tznius in which Boaz asks after one of the women.
Thus, Midrash Rabba discusses her modest actions. (See right, and see here.) The proximate cause of the midrash is Boaz's asking about her.
Thus, the actions described in Rus Rabba are nowhere over the top. She is certainly more modest than the other Jewish women. But still, this is well within the realm of normal modesty, for someone to whom modesty is a concern. There is no claim that she made sure that no one saw her fingers, or that she never spoke such that they thought she was mute.
A bit earlier, there is the midrash in Rus Rabbah which Rabbi Falk referred to, that she was exceptionally beautiful. The midrash is actually a bit more extreme than that. Earlier, it puts her age at 40. But then they cite Rabbi Yochanan that any man who saw her immediately experienced a seminal emission. She was that striking. And this was discussing when she was gleaning in the sadeh.
Now, it is not clear that even within Midrash Rabbah, we should take two separate midrashim, from different sources, together. And the idea of מריק קרי (or in variant girsaot, מקרי קרי) may be in line with the sexual imagery of perek 3.
At any rate, we do not see within Midrash Rabbah that her tznius was extreme; and thus we do not see that her tznius chumras were motivated by the extenuating circumstances. This is a lesson Rabbi Falk would perhaps like to draw, but we do not see it in this source, at least.
There is, however, Midrash Zuta (available here), which Rabbi Falk cited -- that the naar testified they did not see her fingers or toes, and that they did not know if she was mute or capable of speech. I would interject here that just because a midrash says something does not mean that it historically happened. This is the midrash's insight into it, and the historical facts may be different -- or, the intent could well have been to stress that she was a tzanua, to resolve the tension in her character as above, by making her an exceptional frummette. At the very least, there is a dispute between midrash rabba and midrash zuta as to the extent of her modest conduct, and so it is slightly misleading to present only the most extreme, especially when working in other sources from midrash rabba.
Furthermore, there is no mention in Midrash Zuta of מקרי קרי -- Rabbi Yochanan's statement that Rus was "exceptionally good-looking." We do not know that within Midrash Zuta, this was the idea of what was motivating Rus. Rather, this appears to be a retrojection of the attitudes of a certain segment of society, that women must adopt much more than is technically required, because of yeridas hadoros or whatnot, so prevent men from sinning. That is, I think this is Rabbi Falk's take on the midrash, rather than what the midrash actually intended to state.
Rabbi Falk then goes on to attribute a certain view to Rashi -- that her standing to collect the standing grain was an aspect of her tznius, so that she would not stretch and possibly expose something. We will address whether we should attribute this to Rashi, but the fact is that the same raw material was available to him in Midrash Zutah itself, and perhaps he can even fashion a better argument there than in Rashi's commentary. A litte earlier in the perek than the midrash zuta he cited, we have what is pictured to the right. Here it lists other actions -- the ones which Boaz noticed, rather than the ones the naar stated. When she sat to harvest with the harvesters, she turned her face to another direction; that not one of her fingers were visible; that at the time she saw a standing sheaf, she stood and took it, while when she saw a cast-down sheaf, she sat down and took it. Boaz saw these three middot, and thus asked the naar.
Now, there are two ways of parsing this into three different middot. We could take standing vs. sitting as a single item, such that perhaps the tznius is only in the sitting. Or we could take each leading vav as introducing the next item, such that her fingers were not visible because (and note there is no vav) she would stand to collect standing grain.
I am not certain which of these two reading are correct. But if the latter, then Rabbi Falk does not need to make any diyuk in Rashi to claim that the standing was for tznius purposes.
But even so, this is one source, which is late Geonic, which is not echoed by Midrash Rabbah (but rather disputes it), and quite possibly not echoed by Rashi. And this source does not state that Rus was that exceptionally beautiful. And it is a midrash, rather than history, and perhaps was written to serve a rhetorical point. I am not convinced that the idea was that exceptional times call for exceptional tznius. And I do not think that we make use of aggadic material in this way to set halacha, or suggested attitudes for bnos yisrael.
Now let us turn to Rashi. (See here.) Unfortunately, Rabbi Falk dowdifies the quote. And while Maureen Dowd at least puts in ellipses, he does not. He cites Rashi as:
דברי צניעות ראה בה מלקטת עומדות מעומד ושוכבות מיושב כדי שלא תשחה
where Rashi says all that we see to the right. He thus cuts off the first part, which is the question, and jumps from the words divrei chochma to raah bah to malaketet, all without ellipses, which would tell someone that there might be some other relevant fact to be gleaned (pardon the pun) from the context in Rashi.
Secondly, in his translation of the above, he puts the words "[so as not to have to bend down]" without explaining why these words are in brackets. It seems almost as if Rashi does not say these words, but that this is by way of explanation. But looking at the Hebrew, the words are clearly there? Did he first leave them out, and decide to put them in?
At any rate, is Rashi really saying that Rus was being extremely tznius in standing up, as Rabbi Falk asserts? I do not believe so, for several reasons.
Firstly, Rashi does indeed select various midrashim, and he brings them in to his peshat explanation of the text. But he did not cite the midrash of Rus being so stunningly beautiful that she had the aforementioned effect on men. He had opportunity to do so, but in his commentary on those words, he has a different comment. And he also does not cite the midrash zuta that she covered her fingers and toes, such that they could never be seen. Again, Rashi had ample opportunity to cite this, as an example of her tznius. But instead, he cites a much more normal sense of tznius which Rus exhibited, more in line with the idea of tnzius discussed in midrash Rabba. No, we are not free to interpret Rashi based on any arcane midrash we happen to find, when he does not cite that midrash and when the plain sense of Rashi's words seems to be against it.
Furthermore, the normal way of gleaning sheaves of grain is to stand and glean them. And as you glean one, you move on to the next. This is the convenient way of doing it, and this is why all the other women who were collecting grain just bent over in order to collect the low-lying grain. Frankly, to sit on the ground and stretch way up to get to the top of the grain is just idiotic. Thus, if this was what was intended by Rashi's statement, it would not be divrei tznius. It would be divrei chochma!
Furthermore, this entire calculation of what would be exposed as one stretches this way and that, of clothing, as a requirement of modesty, may or may not be true. But this is the idea that is evident in the thought process of Rabbi Falk, and throughout Oz veHadar Levushah -- or anyone obsessed with tnius, to the extent that one comes up with quantifications, in the form of charts and ratios. I am not at all persuaded that Rashi would go through such a detailed analysis of tznius and suggest this in his peshat of the pasuk.
Furthermore, Rashi says why it is divrei tznius -- "so as not to have to bend down." Is is not highly irregular that Rashi would explain what is fairly simple to figure out -- that by sitting, she does not bend down and stick her rear end in the air -- but leave out the explanation of why standing to collect the standing grain is tznius?! Is someone -- other than Rabbi Falk -- likely to figure this out? Rear ends in the air seem more problematic in tznius than fingers, hands, or the top of the forearm. And recall that Rashi did not cite this arcane Midrash Zuta on Rus that Rus' fingers and hands were covered. How is his reader supposed to figure out that this part is for tznius purposes as well? If Rashi really held that this was for tznius, shouldn't he have said something, just as he did for the sitting, such as "so as not to expose her fingers"?
Perhaps this was why the words "so as not to have to bend down" was put in brackets -- he realized the difficulty posed by Rashi explaining the easier one and not the parallel more difficult one. Or perhaps it was just a strange editing error. Or perhaps because he realizes this is Rashi's additional comment, rather than the original words of the source Rashi is citing -- as we will see later -- but even so, it should not be in brackets.
So if so, why does Rashi mention the standing to collect the standing grain? Is this a good question? Not when we see the context which Rabbi Falk omitted, thinking it unimportant. The full Rashi, again, is pictured to the right. See how for divrei chochma, there is a parallel between what she did take and what she did not take:
"that two sheaves she did collect, while three she did not collect"
This was divrei chochma because this is in accordance with the halacha of how many sheaves left together were the gift to the poor to collect. Thus, the form is she collected when X was true, but she did not collect when Y was true.
This same form, it seems clear to me, applies for the divrei tznius.
"She collected the standing grain while standing, but the low-lying grain while sitting, so as not to bend over."
Thus, she stood -- the easier, more convenient thing to do -- where it was acceptable based on tznius to do so, but she did not stand when it was not acceptable based on tznius to do so. Again, the form is that she stood when A was true, but did not stand when B was true.
Thus, if we look a bit at the context, Rabbi Falk's question is not a good question. And his answer is extremely farfetched.
Furthermore, Rashi is not innovating this by himself. He is really simply citing, and slightly modifying, a gemara in Shabbat 113b (see here and here). That gemara reads:
ש (רות ב) ויאמר בועז לנערו הנצב על הקוצרים למי הנערה הזאת וכי דרכו של בועז לשאול בנערהThus, there is actually a dispute between Rabbi Eleazar and a brayta of whether Boaz saw divrei chochma or divrei tznius. Rashi combined the two opinions, such that Boaz saw both of these things in Rus.
אמר ר' אלעזר דבר חכמה ראה בה שני שבלין לקטה שלשה שבלין אינה לקטה
במתניתא תנא דבר צניעות ראה בה עומדות מעומד נופלות מיושב
אמר ר' אלעזר דבר חכמה ראה בה שני שבלין לקטה שלשה שבלין אינה לקטה
במתניתא תנא דבר צניעות ראה בה עומדות מעומד נופלות מיושב
Now, according to Rabbi Falk, the collecting grain while standing was tznius since we know she covered her hands, and sitting while collecting would then expose her hands. But Midrash Zuta, which is the source for her covered hands, is a late Geonic midrashic work. Thus, it is post-Talmudic. And the gemara here -- or rather, the brayta here, is a Tannaitic work. There is absolutely no reason to assume that the author of the brayta was aware of the derasha made in the late Geonic midrash. In fact, it is rather unlikely.
Indeed, if we follow this link to e-daf, we can see Rashi's comments on the side of the gemara. And in explaining the standing, Rashi does indeed comment. But it is only to explain that the shikcha of standing grain is also shikcha, so it was permitted for Rus to collect it. Meanwhile, on the words נופלות, Rashi writes that "those which the harvesters dropped, she collected while sitting, and did not bend to collect them, because of tznius." Thus, Rashi is explicitly explaining the second half of the statement as the divrei tznius. And he chooses, when explaining the source gemara, not to explain the first half of the statement as divrei tznius.
I do not know if Rabbi Falk looked up the source gemara before explaining as he did. Or if he did, how he could persist in offering this farfetched explanation of the standing as divrei tznius.
(The midrash about not standing vs. sitting is brought down from the gemara in Yalkut Shimoni, but this is a later source than Rashi. Or at the least, Rashi does not use Yalkut Shimoni as a source. Still, it is useful to see how this midrash is incorporated.)
Thus, I would conclude that rather than being the correct peshat in Midrash Zuta or in Rashi, it is really simply Rabbi Falk's retrojection of his own tznius attitudes into these earlier sources. This is a pitfall that people in general need to watch out for.