You shouldn't need to emulate the CPU, they are both part of the same ARM family. I think the GBA cpu is a 16.78MHZ ARM7, while the GP32 is an ARM9 which can in theory go up to 150MHZ. I remember I used to run Mac and AtariST stuff on my Amiga, but it was only emulating the subsystems, not the CPU. In fact benchmarking a 68020 Amiga, gave the same speed MAC, with no penalty in processor speed. It was emulating the sound and graphics that slowed it down. I'm not sure, but I think getting the GP to run as an ARM7 at 17MHZ would leave plenty of processor power for graphics/sound emulation. But i'm sure there are plenty of coders who have looked into this.
