For defining features....

One might assume the tip of the nose is the point which lies furthest from the center on +z.  One might look for a nostril material to locate the nose.  One might have the user place a marker object.  One would have to expand to define the nose, overall, beyond that....

For ears, look for clustered geometries on the side of a head which lie outside the basic shape defined by the scalp.  More complicated than the tip of the nose, but it seems feasible....

For a mouth, look for lips, teeth, or tongue materials.  Or look for the area on +z where the normals of neighbor vertices point toward one another on y.  Or look for the place where the mesh turns inward upon itself.

Then if and when any of these are found, find min/max areas for them and calculate basic offsets to use when comparing the meshes.  This could (if workable) remedy the worst concerns about incompatible shapes overall, but then there would still be variance within the defined features to be accomodated.

Complicated.  I'm oversimplifying and being too optimistic, I know.  :)  Right now I'm struggling to understand how to get a vertex delta weight as an inversely proportional expression of one vertex's distance from another vertex.  This project needs a math wiz.

===========================sigline======================================================

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.