The screen/keyboard interface/etc. connects to the Pi Zero via GPIO, not USB, so it should be...I won't say trivial, but entirely feasible to swap the Pi for a uC. Some folks on the Discord instance have already shown examples of other devices connected to the Beepy motherboard (and one person even posted a Pocket CHIP wired to a Pi, which is a nice bit of continuity).
that sounds fantastic, and i look forward to seeing the results
i had an instance of the first blackberry (which was before the 'blackberry' branding; it just said 'rim, research in motion'), and it had the best handheld keyboard i've ever used. the later blackberry keyboards seem quite reasonable too
tl;dr I'm certain this is coming