I very much would like to see this myself. Unless we have some proper implementation for cases like this it could be as simple as simple as specifying, that one adds the type "front" together with a comment of "medium N" (where N is the medium number) should be considered the per disc/medium cover art. Likewise type "track" with "track N" could be used for per track cover art (as it is for example already done here). This can be extended to medium images as well (type media and the comment).
It removes some more detailed uses for the comment field though, for example I really like how the comments on this release specify whether it is a DVD or CD. But we could also introduce a special syntax, e.g. wrap the number strings in [...], and everything not inside [...] should be ignored for automatic evaluation of the comment. So you could wite something like Artwork for track "2 Ghosts I" [track 2], thus the comment provides both the track title (or any other information) in an unstructured way, and the track position in a structured way.