Isn't there some generic tool for handling transmitting keypresses? Then you could use an emulator with local multiplayer (I know this exists at least for two-player) and a video link to have both machines emulated on one host, and one working remotely.
At worst you could have have a limited vnc session or something.
> At worst you could have have a limited vnc session or something.
I don't know specifically about Gameboy but this approach has been tried by PS1 emulators and it doesn't work. There are two main problems, computer and internet speed variations will eventually make both games go out of sync, and if using the clock or some external data for the seed of the game's rng then both players will be getting different things while in game (depending on the game, could be different Tetris blocks, different bonuses, different weapons). I discovered this while trying to play a multiplayer PS1 game over the LAN and the emulator did just that, send keystrokes to the other computer.
As a step to writing a patch for this myself, I hacked in some analysis to try to at least reverse-engineer the GameBoy Tetris protocol (also seemingly unpublished anywhere on the Internet) but never got very far.