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.