A backend is easy to do if somebody compiles a guide on how to add these tags, for example (also if they can have values, special meanings and so on). There are scattered bits from this forum, if anyone wants to link them from here. The XMP spec is huge, and this backend would target only a limited set of tags.
So far, it looks to me we would like to tag:
- exposure compensation (as in Post Deflicker)
- fake aperture of manual lenses
- mark dual ISO files (so you can sort them in post)
- rating?
-

pj = ?
I'd go for a skeleton XMP (like the post deflicker one), and for each of these functions, you'll have to post the XMP snippet required to add that tag in order to be recognized by common software (Adobe? anything else uses XMP files? are the tags universal or app-specific?)