By the way, the IR led remote thing would be quite simple to get working (at least as simple as the 83+ one) but it would require more invasive external hardware (on the level of soldering to the dock connector, like sound for Lua). Two way would also require an IR receiver thingy (

don't remember its name atm, not expensive) so you can recieve IR signals. This would also be helpful in figuring out remote codes.
Also, the GBC link port (not the IR one) was quite complicated. It supported up to four game boys at the same time. it would be quite difficult, I'd imagine, for very little reward.
@Spyro I doubt it would be difficult at all, for someone experienced in making emulators (read: not me) but I doubt many would be interested.