Ucon64 also supports Game Genie and has source code
for it in the "src\patch\gg.c" file.
As for a "cheater", I think and Arduino would be fast enough to replay TAS controller input (youtube search tasbot) through the controller port, but not fast enough to change ROM values in real time as a Game Genie would. Maybe an FPGA would work for that?
There's also the Pro Action Replay, which can modify RAM values. I'm guessing rather than just modify the ROM data bus (is that how the GG works?) It actually runs code which can update the RAM values in the SNES. It even has a feature where you reset the SNES, and it will search through the memory and find codes just like the cheat search in a SNES emulator.