Hacking Battle Backgrounds

Considering that each Background is constructed by placement rather than by just drawing an image, I'd say it's pretty difficult.

I suspect that for the best results, you'll want to use a background that is also made up of small tiles. So something like NES Mario may in fact be a good test subject. Similarly, if you just want to do small edits to the 'edges' of the background, it seems pretty doable.

But completely changing the background looks like more trouble than its worth at this point. Maybe someone will hack the assembly function for the BGs and make an editor at some point, though?