This is one of the more plausible ideas I've seen in a while, though I see a few issues with it. The biggest problem is definitely going to be input. If you can work out how to use a DAP with around 5 keys to emulate a device with over 50, please do tell.
There are already multiple Z80 emulators in Rockbox (pacman and rockboy). Modifying one of them to emulate TI hardware shouldn't be too hard, as long as there's someone there to do it, but making it usable might be a challenge.