Was in the interior of the country, where they would have been hiding-centuries later, under the Muslims, there are examples of Jews assimilating to the Coptic speaking communities, as shown in the Cairo Geniza.
But the earliest Geniza documents come from the 9th century or so. Is there any reason to suppose Egyptian Jews would have used Coptic amongst themselves in the 1st century?
The problem is that Coptic wasn't written at the time, and hieroglyphics and hieratic were associated with the pagan Egyptian cults-the reason why they died out with the conversion of the country. When the Romans came in in 30 BC, they immediately implemented their policy of requiring Greek in legal documents etc., which led to the rapid decline of the popular written form of Egyptian, Demotic. All of which causes problem of positive evidence of Jews using Egyptian at the time (there are earlier Jewish graffitti in Demotic along with Aramaic). Ostracha, however, attest that Egyptian, at least outside of Alexandria, remained the language of Egypt, and even the most Hellenized Jew, Philo of Alexandria, shows some knowledge of Egyptian (though it also shows he did not himself speak it), and we have no evidence of anything remotely like the later ghetto shtetl, walled off from society at large.
We do know, thanks to the Nash Papyrushttp://en.wikipedia.org/wiki/Nash_Papyrus
that Hebrews in Egypt, despite the homeland of the LXX, used Hebrew, although a text closer to the LXX that the MT.