"The Poser MorphBrush uses a custom channel that applies the morph before the joint deformations."

Unless things have changed since P6, all the morphs (meaning the deltas for the vertices) get applied before any rigging affects (JCMs control how much of the morph gets applied as a function of the joint).

My approach is to tie a local coordinate system to each vertex so I can use the base and posed objects to map all deltas on the posed mesh back to the unposed one (so a delta-x when the arm is down becomes a delta-y in the default pose when the arm is up). This doesn't have anything directly to do with the rigging, it is just a way to clean up rigging issues after the fact (using ZB).

If DS does the equivalent of this, I should look at it. I don't suppose it lets you export figures without munging the vertex order (like Poser is famous for)? Dealing with sliced & diced mesh is a pain I can do without.