Bo #3:

A though occured to me about Daat Zekenim's mention of the fact that the Roshei Tevot of the 10 plagues on Moshe's staff might have clued him in to the idea that the next plague would be locusts.

A problem with this is that Moshe does not seem cognizant of the last plague. Consider Shemot 10:28-29:
וַיֹּאמֶר-לוֹ פַרְעֹה, לֵךְ מֵעָלָי; הִשָּׁמֶר לְךָ, אַל-תֹּסֶף רְאוֹת פָּנַי--כִּי בְּיוֹם רְאֹתְךָ פָנַי, תָּמוּת.
וַיֹּאמֶר מֹשֶׁה, כֵּן דִּבַּרְתָּ: לֹא-אֹסִף עוֹד, רְאוֹת פָּנֶיךָ.
"And Pharaoh said unto him: 'Get thee from me, take heed to thyself, see my face no more; for in the day thou seest my face thou shalt die.'

And Moses said: 'Thou hast spoken well; I will see thy face again no more.'"

And then immediately following, in the next pasuk, Shemot 11:1
וַיֹּאמֶר ה אֶל-מֹשֶׁה, עוֹד נֶגַע אֶחָד אָבִיא עַל-פַּרְעֹה וְעַל-מִצְרַיִם--אַחֲרֵי-כֵן, יְשַׁלַּח אֶתְכֶם מִזֶּה: כְּשַׁלְּחוֹ--כָּלָה, גָּרֵשׁ יְגָרֵשׁ אֶתְכֶם מִזֶּה.
And the LORD said unto Moses: 'Yet one plague more will I bring upon Pharaoh, and upon Egypt; afterwards he will let you go hence; when he shall let you go, he shall surely thrust you out hence altogether.

and then in psukim 4 and 5:
וַיֹּאמֶר מֹשֶׁה, כֹּה אָמַר ה: כַּחֲצֹת הַלַּיְלָה, אֲנִי יוֹצֵא בְּתוֹךְ מִצְרָיִם.
וּמֵת כָּל-בְּכוֹר, בְּאֶרֶץ מִצְרַיִם--מִבְּכוֹר פַּרְעֹה הַיֹּשֵׁב עַל-כִּסְאוֹ, עַד בְּכוֹר הַשִּׁפְחָה אֲשֶׁר אַחַר הָרֵחָיִם; וְכֹל, בְּכוֹר בְּהֵמָה.
"And Moses said: 'Thus saith the LORD: About midnight will I go out into the midst of Egypt;
and all the first-born in the land of Egypt shall die, from the first-born of Pharaoh that sitteth upon his throne, even unto the first-born of the maid-servant that is behind the mill; and all the first-born of cattle."

Moshe did not know of this plague before Hashem told him there would be another plague, yet he could have seen the ב on the staff and known there was one more to come, and so would not have confirmed to Pharoah that he would not see him again. As it was, Hashem had to make an exception and speak to him within Pharoah's court.

More than that, one can ask what was Moshe's source that the next plague would be Makat Bechorot? Hashem just said another nega, but did not specify what it was. You cannot refer to the staff if Moshe did not even know of the existence of the next plague at first. And, the drasha of ספר which worked for locusts does not work here. Perhaps you can find another derasha. The easiest seems that Hashem told Moshe but that fact is not mentioned in a pasuk, like I suggested above.