Managing secondary files is a real need, but can get deep and complex, or not, depending on the internal structure of MC's database. Can it do multi-table relations?
With even a loose one-to-many "relational" approach, lots of doors open. Assuming the visible table of the primary media files (music, etc) is the primary table, there might be a secondary table contains info about secondary files: types, names, locations, and what primary files they relate to. If every record in the secondary table is related by a key to the primary table, when a file location is changed in the primary table, its secondary files can automatically be identified and changed, etc.
Except for the related-by-key cross-links (and associated plumbing) the secondary table could be just like the current primary table, therefore MC's various tools could be applied to it.
MUSIC FILE in primary table
related by permanent key to any number of records in secondary table, such as:
-- front cover art .jpg
-- back cover scan .jpg
-- lyrics .txt
-- liner notes .txt
-- comments .txt
-- whatever
When a library (primary table) is opened, so is the secondary table in the background. Mostly a library's secondary table is updated automatically by changes in the primary table, but it could be accessed directly via a tab or menu option. Just protect the unique keys in each table that form the relationship (probably hide them).
Another benefit of a two-way link between primary and secondary files would be better handling of orphan "cover art". If the secondary table has contents not related to the primary table, it's easy to identify as orphaned. I use a central "coverart" folder as better for my needs than the other two methods, and right now, it has many, many orphan cover files that I "know" aren't targeted by any music files in the primary database, but I can't easily "prove it". I'd love to identify orphans and clean up the central folder.
Having a two-way table relationship could also help automate switching among cover art options such as Folder.jpg to Artist / Album to central folder and back.
This also could make it easier to alter the filenames and locations of secondary files and not break (or even touch) the primary media files. Just update the secondary table.
I know, it's easy to dream up stuff that is a big job to build ...