On the one hand I understand the wish, on the other hand as stated in a post of mine Utilize media metadata Date and Face-Tags in import and references the preferable way of syncronizing in my opinion would be to have the origin of the faca-tags in the foto and not in the reference of the foto in gramps. If someone would want to go the other way from the reference to the origin in the foto the most conceptually correct way would be to write those tags via a gramplet into the xmp data of the foto directly.
The goal of such syncrozisation in my view would be:
- have the tag only in ono place
- the one place holding a tag for a foto should be the foto itself
- syncronize this information with usage of fotos in gramps