"I assume the exported .obj can just be imported into Poser, treated with the morph brush, exported again, then re-loaded using the script?"

Yes, you can do this.

While the MorphBrush in Poser8 / Pro 2010 has gotten a lot better, it still has problems smoothing extreme joint deformations.
So morphing a (welded) object is a lot more accurate with it than morphing a "live" mesh.

So you just grab the posed and welded mesh that the script exported, re-import it into Poser, morph it using the MorphBrush, and then export it again as an object so that the loader script can convert it back into a morph.

BTW, the morphs that this script creates are "ordinary" morphs.
So, unlike the "special" MorphBrush created morphs, which need a special morph channels and can't be injected easily, they can be distributed and injected and used by owners of older versions of Poser just like any other morph can.

Simply said, this script can do anything DAZ ' MorphLoaderPRO can do (Which you can only get by buying the  PRO Rigging tools bundle), but it does it easier and faster and especially MORE ACCURATE.

The fix morphs don't need to be "re-fixed" in Poser.
You will get EXACTLY the same shape on the actual figure that you sculpted on the welded object.