Even if it would be possible, someone has to unlock DSi mode outside of running official DSi programs. Then someone would have to actually create a GBA emu for the DSi, or hope the DSi still has the GBA hardware in it to run them.
Another thing is that the DSi STILL doesn't really have enough RAM for many GBA games. It only has 16megs and some GBA games are 32meg, so you would be limited to games of 16meg or less.
My guess is that if any GBA games appear for the DSi, they will have to be re-worked to run in the limited memory of the DSi and will be made available through the DSware store......as in, you will have to PAY for them.