Basically making it so that instead of having to choose between Boss1/Boss2 you could browse for a music file similarly to how you pick level music.

Would that be possible? Part of me feels it would already be implemented if it was, but I figured I'd shoot my shot just in case.
Otherwise maybe make it a MLLE-exclusive feature if it can't be done in JCS? It would still create an automatic script similarly to palette/tileset changes but it would still be nice as a QoL feature.