The thing about the GBA Movie Player's support for actual GBA code is that it must all fit in RAM. Basically just things that are made to work in multiboot mode. So even though you can find a boatload of GBA homebrew games and such, only a fraction of the small ones will work. That being the case, testing to see if they actually work is quite a pain, and there are probably many things that owrk that I don't know about. Looks like I've got a calculator, Yahtzee, Minesweeper, and graphically/aurally weird Tetris game "Tetanus on Drugs" sitting on mine.
As I mentioned in one of the PSP threads, the Game Boy emulator Goomba works well for small-ish ROMs, so the combined Goomba + ROM file is less than 256K.