MC will automatically use the "abum artist (auto)" field of (multiple Artists) for albums that have multiple artists. This is a really cool thing that works very well. Keep the artist tags filled in correctly, add them all to the same 'make-believe' album and MC takes care of the rest. As far as MC is concerned, I have an album called "Single Tracks" with over 500 tracks on it!!
The only thing to remember is that when doing your rename from properties, use the [album artist (auto)], not the [artist] field in the naming string. What I do for regular albums, is to use the Artist - Album for my directory name, then track number - name for the file name. For multi artist albums, I add the artist into the filename. If you used this method, you would have a directory called (multiple artists) and inside that would be a directory for each of your multi-artist albums, and inside each of those, you would have the files. Is that what you're after? The rename files from properties string would be...
Directory Naming....
[album artist (auto)]\[album]
File naming....
[track #] - [artist] - [name]
-marko.