Actually, I was thinking of creating another version featuring a real 6502 with RAM and ROM, and a separate AVR doing nothing but video signals on the side. This can be done using dual-port RAM for the video chunks of the memory, allowing the AVR do its magic without worrying about the computer's ...

Looks like we had the same idea, even about the same time.
http://jmp.no/blog/apple-iie-on-avr-with-video

I started with porting my 6502 emulator to the ATmega128, which worked fine. I pushed it a little further and tried to emulate the video simultaneously, but that was a bit too much. Emulating ...