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[TheOldPresbyope]

That part's easy. ID3v2, for example, has the POPM field. (Even if it didn't, it's possible to define a custom field.) Most every tagging scheme would allow defining a counterpart.

The hard part is getting all the backend clients (particularly MPD's plugins in our case) to agree its use and implement it and then writing frontend clients which expose it to the user.

Regards,
Kent