I think this idea has something :)

Probably the idea should be reworded to come to an easier solution.
"The possibility to parent a prop to a group of polygons of a certain actor".

I changed to a polygon group because such a group can easily be handled with Posers Group Editor and selected polygon groups are rechable via Poser Python.

One method of resolution:

*If a prop is parented to an actor and group-parented to a predefined group,  the (computed) centerpoint coordinates from the selected polygongroup are saved as a reference point.

On each WorldspaceCallback the actual parentgroups centerpoint is computed. The child prop is moved by group_saved_startingpoint - group_actual_position (if they differ) and group_actual_position is saved as the new group_saved_startingpoint (all local coordinates, because the prop is also parented to the actor).

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