I don't have a solid solution for this one, but I thought I'd make it visible in case anyone gets ambitious.

Using net.fabricmc.loader.impl.launch.FabricLauncherBase in FabricRemapper (Import FabricRemapper.java#6, usages FabricRemapper.java#34 FabricRemapper.java#38 FabricRemapper.java#41) causes Rhino to fail without fabric loader even when a drop-in replacement for FAPI's interfaces (QSL) is provided.

Sample crash log

It's used as the source of the mappings in use, so there'd need to be a whole usable replacement for getting those. No idea - But I might as well make it known and toss it out to the quilt team/community to see if they have anything useful.

I guess ill just disable remapping if there's no FabricLoader class for now

This should be fixed now, re-open if it's still an issue