If you are willing to change your disc numbers you can use 1nonplayable track with the box set cover art set as disc 1, then the real disc 1 gets a tag of disc 2, etc. You can create a new tag with the real disc numbers fotr dsplay.
I am not sure what happens if you just leave the disc # blank with a nonplayable track with the box set cover art. Unfortunately I do not have access to a computer to test.
If MC allowed a disc # of zero this might be an easy solution to an ongoing issue.i