If you take apart a classic gameboy cartridge it basically just has an addressing chip and a ROM chip on it. Would it be feasible to take out the ROM and instead connect the addressing chip and the rest of the addressing pins directly to a set of SRAM chips?
For example you'd have a circuit powered by the arduino that would write into the SRAM chips connected to the gameboy through the cartridge and when it's done you could turn on the gameboy and it would read from the SRAM banks. This would mean two circuits have only the SRAM chips (and the relay) in common. A relay would shut down the arduino circuit the moment the gameboy is turned on so there would not be double the voltage going through the whole circuit but also the SRAM would never be without power and loose it's data.
Is this at all feasible? Am I overlooking something? I reckon that a capacitor should smooth the flow during the switch from one power source to the other. The other concern I have is wiring the SRAM to the cartridge. Would it still be responsive enough for the gameboy's processor? From what I've calculated the SRAM should be fast enough but I'm worried about the latency the wiring will add.