Was just thinking about this as a possible feature...
Part 1: Have a configurable option on each server, which would either allow or prevent the ability to upload database catalogs to a central Madsonic server somewhere in "the cloud", where site owners can upload their music catalog (song titles, movie titles, etc...).
Part 2: On the client end, have an option for each song to "search" the database catalog from "Part 1", which will then show results of who else is running Madsonic (and has the feature enabled), and has the same (or similar, for a Pandora effect) music or videos, and gives the end user the ability to listen to that song as "guest", similar to the "share" option that's already in place.