"All that needs to be done to set up the JCMs in the shirt is basically split morph and reverse deformations for the rotations. No need to set up a dependency to the rotation, the crosstalk in M4 handles that."

I meant subtract here, Poser semantics.

Basically you want to subtract just the deformations derived from the rotations leaving just the changes from the actual morph fix itself. I suppose you can use convert to post/pre transform, I've always found it to be a bit confusing and wonky. I subtract by zeroing the rotations, leaving the morph fix active, and exporting that result as FBM. Reapplied it will just be the fix itself, the rotation deformations are subtracted