I recently began a MU/BuddyPress site using the Studio plugin. As part of this I am running a small directory of restaurant listings using phpmydirectory. Assuming that Directory in its next release works as an addon to my root blog, would its functionality be such that I could use it to replace phpmydirectory, which I was dreading rethemeing to match the rest of the site, and which I would be happy to move on from. You can see the directory here -- http://www.goutiful.com/directory/
Did you take a look at phpmydirectory when architecting Directory? If not, it's well worth a look, as it has been around for some time and has been steadily enhanced by its developer. However, integrating it with WP seems next to impossible.