[ThomasWinwood]

You make it sound like there's a machine you can put a Game Boy ROM into and get out a disassembly, which is kinda true (https://github.com/mattcurrie/mgbdis) but it doesn't automatically split out data blocks or anything like that - it just tries to crawl the ROM and disassemble any code it can find. It's certainly not "effectively no additional work" than making targeted alterations to the binary and documenting your work.

And that's before you get to platforms where most if not all games are written in C - I question whether a mere disassembly of a game like Pokemon Emerald would even be useful to anyone, whereas the pokeemerald decompilation (https://github.com/pret/pokeemerald) is clearly useful but was a heck of a lot more work to produce.

> That's a hurdle, but not an insurmountable one... unless the system didn't even support banking. (Are there any systems that didn't?)

Depends what you mean by "support". I don't think any system has a built-in mapper - they just assign a chunk of memory space to the cartridge bus, and if your game is larger than that chunk of memory space you include a mapper on the cartridge. Nintendo provided standard mappers for machines like the NES and Game Boy because it's very hard to include a substantial game in the wedge of memory space you get on the processors in those machines, whereas only one game on the Genesis/Megadrive needed one.