To support MSX2, you need to emulate at least 64 KB RAM, and if I don't get me wrong, in this case, all the GBA RAM should be used, what means no way to support correctly VRAM, keyboard, ...
I think it has 256kB + 32kB of RAM. Having said that, that is indeed cutting it close, especially considering it only has like 96kB VRAM. (iirc)
"I'm trying to save 128kB of the GBA RAM so you can send the emulator with a game to other GBAs without flashcards (and for those with a GBAMP). I will eventually add 64kB RAM to the emulator in the future but I rather wait a bit so I don't have to update 2 versions all the time."