It all depends on how people at Pekaro Software did the GBA version, which was made available only recently (perhaps there was a beta version before? I don't remember):
If it uses data files from the PC version, it may be possible to simply replace the files, which is basically what the level packs for the PC version do if I understand correctly.
I am pretty sure I have seen some GBA homebrews which came with sources and a make.exe.
So if the people at Pekaro Soft. share the sources and it is can be compiled easily, then there is a chance to use the level packs.
I have not tried the GBA version yet, but I suppose there have been trade-offs to make a portable version.
I suppose that custom graphics would need to be resized/reworked to be put in a GBA version.
Perhaps only replacing the level files in the sources would work, letting you play the custom levels with the original graphics (not sure what would become of sounds and musics).
I know there are some tools to explore NDS roms and extract some assets and even replace them. Not sure if that is the case for the GBA.
So I believe that unless the sources are released there is little chance to play custom levels on the GBA. At least, there is the original T2002 to play.
Edit: I just tried the GBA version with not much luck.
I don't own a GBA but I tried:
- on a DS (lite using WoodR4 which normally runs GBA games and homebrews fine). The game starts but get stuck at the GBA logo screen (with the Nintendo logo garbled).
- on the gPSP emulator. The game runs very slowly. Perhaps there are some settings to tweak but I highly doubt there will be a massive change.
Checking the readme, it mentions:
2) SYSTEM REQUIREMENTS
T2002 GBA runs on Game Boy Advance emulators, such as Visual Boy Advance (http://vba.ngemu.com).
No mention of an actual GBA. So I wonder if the game is actually playable on a real GBA. Perhaps it needed too much power, with emulators being the only platforms able to run it a full speed, which would explain why it was not released until recently.
Anyway, if someone can manage to make it work at a playable speed on a GBA, I'd be very interested to know.