I haven't tried it yet, but what MOD (Tracker formats like .mod, .s3m, .xm, .it, .mtm, .669, .stm, etc) formats are supported as music lumps, and how well is it supported?

None, currently only midi and cd-audio is supported. Streaming audio is planed and is one of the priorities for further versions.

Ah, okay. My CheapHack engine (which is also GPL) used MikMod for mod support, worked great though you at least have to set a couple of flags on playback for it to play properly (as it defaults with horrible echoing, etc) I think Mikmod works for DOS as well, i've seen a dos mikmod player once, or maybe Allegro has some mod playback functions, can't remember.

Yes, most likely that's what I'm going to use.