UFID is also in id3v2.3.
I don’t actually like the UFID definition - where the Owner Identifier field is supposed to be used for an email address or a web page containing an email address that people can used to email questions about the database. (Any volunteers for handling the millions of emails that this might generate?? )
However, if we ignore that and use the Owner Identifier field for either the MuB home page or a file-specific URL, then there is no reason why Picard shouldn’t read/write/populate the UFID tag.
I would put the Owner Identifier as either the MuB home page or the URL to the Release / Recording web page, and the identifier as the Track / Recording MBID (not sure which of these options would be best).
The other question is whether we should overwrite an existing UFID tag if it holds information for a database other than MB (given that we already have the TXXX fields as the primary indexes into our database) - my gut reaction would be to add support in Picard for reading/writing UFID, and use one of two plugins to populate it - one plugin to populate it regardless of existing contents, the other to populate it only if it doesn’t have a different database already.