Im not the Smartest guy around but, would it be possible to do it if someone made a new custom ds firmware that could be installed kinda like flashme that could allow the ds to run GBA on slot 1?
Theoretically I suppose it might be possible. I think it more likely though that someone might create a GBA emulator that runs in DS mode, but the fact that it hasn't happened yet makes me think that neither task is as easy as it sounds.
Anything is possible, but IMHO there's no money in it since there are slot 2 cards. What would be the point in trying to do it, other than to say that "you can". Besides, if it could be done, don't you think that Nintendo would have done it and included a slot 1 adapter with the DSi