Hi,
Not sure what exact case you want to archive, but you are right that Multi Value fields are not searchable for containing more than 1 Value, so I added it as https://www.ventismedia.com/mantis/view.php?id=15668

For Other case there is a feature in MMW that can list you all possible compilations You can access it under File -> Maintain Library -> Create Multi-Artist Albums and disable rest of options.

I'm trying to find all the compilations I have in my 120k+ library that I haven't assigned "Various Artists" as Album Artist yet. The Magic Nodes and Files To Edit features don't seem to do what I need, which is something like "Albums with 'Album Artists != Various Artists' AND # of Artists > 2". Any suggestions for how this could be accomplished somehow? My SQL Fu is non-existent.