MSU-1 Hacks

Can it be made to only effect out of battle? Or is it a all or none type deal?

There's no restriction. You can have it only available for out of battle songs or have the SPC-700 play the default songs if PCM files are not found in the library.

I've already attempted to implement it, if crudely, and while it seems to work out of battle, in-battle it corrupts graphics since it hijacks the NMI routine.

The guy on the Qhimm forum has a proof of concept video in battle: https://www.youtube.com/watch?v=vKgSqG2Tbfc. I know it's just not a swap and several event commands would need to be edited. The problem seems to be with fading out tracks, but hey, I have nothing to lose if I attempt it.

There's also this guy who made the opera scene: https://www.youtube.com/watch?v=tGlIKAukm1U