Erm, couple problems with that. First, you'd need a HUGE flash cartridge, and the biggest made is 1 Gb (128 Mega Bytes). Second, the GBA has a 33MHz processor, which is dreadfully slow for running one emulator, let alone one on top of another, and a system on top of that that needs a 233MHz processor or so. I'll put it this way: if you managed to get around the fact that there is about 1/32 the RAM required, video support that would need to be severely hacked to display a picture, and the whole space issue you could have a GBA that takes a few months to boot and is slower than the ENIAC.

To sum it all up, don't try it unless you want your GBA incapacitated for a year. If someone could manage to port (not emulate but port) System 7, that might be possible, but emulating OS X

You'll end up like the guy who installed OS X on his Quadra through PearPC on Linux.