And that also brings up another thing. i could make a small little lightweight app you could run that would set the hardware to boot from the 64k rom or 128k rom.

because if i use an ATmega256, i could fit both ROMs in just the flash area. and use a control byte written into its eeprom to decide which one to boot from. simple enough, i would just have to give that control byte its own address space on the macintosh buss. just write a $01 or a $02 into that address space will determine which ROM to boot from.