Interestingly, this core seems mostly complete. After some slapdash porting, Ash managed to get the the thing to boot on mister using bram as ROM storage. It will load BIOS and play a homebrew 'mario pong' demo successfully. It wants 24 bit memory addresses for ROM accesses so I tried to use DDR for convenience trying to get larger ROMs to load, but it seems the memory controller implementation of the GBA maybe needs a bit of reworking. Very promising though. The github linked in the OP has a 'final report' PDF with a lot of technical information about the GBA and the state of the core if anyone is interested in pursuing this project.