There are plenty of apps that will pull from external sources, but they will operate in their own way, pull data according to their own rules and specs, but not operate on MC fields directly. I wanted to create some scripts that could pull data from discogs, amg, etc., and have them populate MC fields in the Library (either directly via MCWS or via MPL import) rather than update external file tags and have MC read those tags.
A couple of features were important to me and other users:
1. user-defined MC field names
2. credits mappings
3. selective field assignment (via inclusion or exclusion rules)
4. ability to populate MC metadata pre-rip (fills metadata values for the CD entry in the CD database, which carries forward to ripped tracks)
I'm not sure if this addresses what you are asking about, but maybe its a start.