(More or less copied from a PM to Iceknight.) 22c0c8 is the pointer to the 1st stage music. I used this as a base. 22c170 is the pointer with the 1st unused music. Change the bytes in the base pointer to match these to get your unused music in its full glory. More pointers: 22c178 22c180 (Afterburner if I'm not mistaken.) 22c1c0 22c1c8 22c1d0 22c1d8 22c1f0 (I'm not sure if this is unused (It's even in the OST), but it's here for reference.) It was easy to find using gba2midi. I hope you all enjoy.
I hate to double-post, (couldn't edit) but these offsets only apply to Gunstar Future Heroes. Super Heroes' offsets are around the same location.
Lots of older Sega games' music is stuffed in there. IIRC, there's 2 Altered Beast tunes, the Afterburner one, and something else I can't quite place my finger on. (ICEKnight mentioned Thunder Blade, I believe.)