Okay, I think I've got a script that's streamlined enough to unleash onto a courageous few beta testers - a.k.a. whoever still follows this thread.

Get the code as a ZIP file from here or for git users, clone from https://github.com/odf/pydeltamesh.git.

Vague instructions:

A) Exporting the unmorphed mesh:

In Poser, load the figure, set to unimesh and the desired subdivision level, then export as OBJ with "As Morph Target" checked (sorry, no post-transform support at this point).

B) Making the morph:

In your favorite tool, import the OBJ from A), sculpt it to your heart's desire and export as OBJ again. Important: make sure to keep the vertex order and also (for now) the disconnected vertices intact.

C) Loading the morph:

Back in Poser, load the figure with the same settings as when exporting and make sure it's selected, then start the script via "Run Python Script" in the file menu. The path to use for the script is pydeltamesh/poser/loadSubdMorph.py within the unzipped folder.

The script asks for a name to give to the morph (if none given, the OBJ file name is used) and then pops up a file selection box where you can pick the OBJ created in B). After that, my Poser just seems to sit there for a few seconds before even showing a progress spinner, but eventually you should see a text box pop up with some diagnostics, and a dial for the new morph should appear.

-- End of vague instructions --

That's it for now. Undoubtedly there will be many bugs, so please be gentle with me.

-- I'm not mad at you, just Westphalian.