I have made a premilinary schematic for the transfer cable, check it out.
wwwk.heltech.edu.hel.fi/koti/sepi/nes/ltp2nes.gif
wwwk.heltech.edu.hel.fi/koti/sepi/nes/transnes.txt

Parity bit can be used in PC to NES connection, this actually depends
more from software.

In case of NES to PC connection, we do not have that many ports available,
at least in the desing that i have been working on, so byte checking in this
case is probably our only shot.

Downside in the current desing is that PC or NES does not react if received byte is incorrect, so you must manually erase the incorrect bytes.

By the way, how complex cartridge back-up units are? i mean, if we could fit this device inside GameGenie unit, would it be possible to read ROMs from the the cart and transfer them to PC? i know this sounds kinda crazy...