The script is available, but the outcome for this kind of problem really wouldn't be certain.  Ultimately, I think it would be easier to try my first idea.  If the unwanted motion was due to export without the Morph Target setting, it should (hypothetically) by constant in all the morphs and also easily recreated for a subtraction morph.

The Python script is here, however, and it shouldn't require any special knowledge of the user.  The script presents several different ways to trim from a morph some of the deltas but not others.

http://www.the.cage.page.phantom3d.net/TDMT_Match/morph_trimmer1b.py

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Cage can be an opinionated jerk who posts without thinking.  He apologizes for this.  He's honestly not trying to be a turkeyhead.

Cage had some freebies, compatible with Poser 11 and below.  His Python scripts were saved at archive.org, along with the rest of the Morphography site, where they were hosted.