Directory or catalogue tag

[SNQN]

I have enjoyed MinimServer for some years now with a fairly mixed music collection.
To the classical part of the library I have added a Directory or Catalogue tag to each title eg. "BWV 0971" for J.S Bach.
I use this tag for indexing and for information on the "Now playing" screen on the Control Point (BubleUPnP).

But I can't figure out how to do this: I would like to display the Catalogue info only on the relevant top level ie. for the Matthew Passion with the Album tag (using Album.display.format), for the Brandenburg Concertos with the Group tag for each of the 6 Concertos and finally with the Title tag for individual compositions on an Album not being part of a Group.
I can display the Catalogue tag for instance for a Group but I also get it on each individual Title in the Group.

Another thing: as indicated I have inserted leading zeroes in the BWV numbers to achieve a correct sorting in the list view. Is there any way to avoid this except from splitting the tag into two: a text tag "BWV" and a number tag "971"?