Thanks for the suggestion. That lets me change the field contents (i.e. "7a8dbe84-f4c0-4457-bfa3-edced1f8cde0" ↔ "7A8DBE84-F4C0-4457-BFA3-EDCED1F8CDE0"), not how the frame name is stored.
Basically this is the same as Writing back MusicBrainz IDs in all Uppercase? , just with another frame.