This is a plugin for Picard designed to add MBIDs for all other roles where possible (remixer, composer, label). It tries to use the same naming convention as the original tags.
Standard Picard (see picard/mbjson.py / ARTIST_REL_TYPES for the list) tags are mapped to musicbrainz_role_id.
Performers (instrument, vocalist, other roles) are mapped to musicbrainz_performer_id as a multi-valued field.
Each item in the tag is in the form artist-mbid (role), where role is one role.
For example, if an artist A has two roles, piano and guitar, you would have musicbrainz_performer_id be A (piano); A (guitar).
In addition, ensure that performers are standardized in the form "artist (role 1); artist(role 2); artist(role 3);", rather than "artist (role 1, role 2, role 3")