On Rate Your Music, as far as I know, it's possible to put any entity in any list.
A great example of this is the RYM Ultimate Box Set, which compiles the "essential" artists, labels, songs and releases in a genre. I've also seen this being used on lists that are about an artist/label, but is not a list of artists/labels itself (e.g. a "Best releases from label" would include that label too so it shows up on the label page).
Two other use cases I came up with are "artists and labels to check regularly for new releases" and "artists, labels and venues that maintain a good history page of events".
Since different entity types have different tables I imagine it would look similar to our current release group collections, where there are separate tables for different release group types.