Thank you for providing this additional information.
Unfortunately, neither the _fields nor the saving of multivalue fields are planned for the near future.
The effort required to add and maintain these rather specialized and user-defined fields is currently too high.
Of course you can copy my websource script code and adapt it to your needs.
Or you can use "MusicBrainz Picard" for these specific sort fields.
AFAIK, MB Picard also supports multivalue fields, so you could implement both suggestions.