This works, but I'll need to test across other games which hopefully will not involve more than sorting out correct button mappings. I noticed that for scummvm, its only backend options are for SDL2 and therefore will not work (as far as I can tell) with xboxdrv.

That's because ScummVM is already a SDL2 application and there's no need for a compatibility shim like sd12-compat.

It's looking to me as if xboxdrv may be a bit of a dead end when it comes to button remappings for non-libretro emulators in RetroPie. Can such remappings be done in SDL2 or is there another option that could be applied system-wide?

Not dead, just not widely used. xboxdrv itself is mostly feature complete and there's little development nowadays, since compatibility with Xbox controllers is offered OOB by Linux kernel modules (xow, xpad, xpadneo).

There are no alternative re-mapping solutions in RetroPit, though anything that works in Linux and without X11/Wayland could work.

Mapping keys for Player 2 in atari800