[nickpsecurity]

"The former ought to be possible, especially if I'm targeting C or LLVM (some bits might get a bit rough, but most of Scheme is pretty easy to implement)."

That's what they did originally. Should work.

" but the GBA is a really well-defined piece of hardware"

Then do a PreScheme wrapper on it supporting inline or calls to assembly for performance-critical routines. See if you can make the primitives on bottom close to how hardware works for efficiency. The AI research of the 80's indicate it might be able to handle a board game.