i think i have mentioned this on irc before, but i thought i'd post it here to keep it in mind.
something like "album properties" would be nice, i know it partly exists already (when editing the properties of multiple songs), but what would be nice would be an album-contextmenu for songtree-headers. (meaning: ability to set albumart etc.)

It's not what you're asking, but I use this little plugin that adds set/fetch cover options to song context menu. At least it makes album art management easier.

Added in my development branch. But remember, this is not the official gmb repository.