It may make sense to allow barcodes and cat# on mediums.
I've seen that in several contexts now:
1) Cases like: https://musicbrainz.org/release/5508c513-970a-4e59-94d1-902e53ac2b64
This is e.g. a box, which contains CDs which each has it's own cataloge number printed on (in addition to the "set cat#", i.e. the cat# we in MB store for the whole release).
So these are really dedicated mediums with dedicated cat# for such multi-medium releases.

2) compilation boxes, which actually just contain a set of jewel cases the were separately released (with their own barcode and cat#)
OTOH, a better solution here would be, to perhaps allow to "link" a release with the mediums from another release.
E.g. IIRC there was a Lord of the Rings soundtrack box, which was just like that... the single albums + a cardboard box.
So the release for the cardboard box would just contain pointers like
<MBID from the respective LOTR 1 release>, medium 1
<MBID from the respective LOTR 2 release>, medium 1
<MBID from the respective LOTR 3 release>, medium 1