Hmm...I've been looking at getting my son an Arduino clone kit (ATMega based). Maybe that would be a good platform for this kind of thing since they are easy to reprogram.
I have no desire at this point to go C64 keyboard-to-PC, just the other way around, so that might simplify (?) things. There are hacks to do this through the cassette port with a .PRG loaded, but I'd rather have a transparent hardware solution.
SX-64, (2) Breadbins, 64C Rev E
1541-II, (3) 1541 (Alps), (2) 1541 (Mitsumi)
Koalapad, Muppet Learning Keys, various joysticks
64HDD running on DOS 7.1, OnNet PC/TCP, NFS mount off Linux server