If you’re using Vista, I’d suggest using creating a symlink at “My MusiciTunesiTunes Music Library.xml” which points to the Firefly copy instead of hacking the “My Music” path. This seems less risky to me.
http://en.wikipedia.org/wiki/NTFS_symbolic_link
You could also try using a junction if you’re using an older version of Win but it is less likely to be safe or even work.
Finally, keep in mind that with any of these solutions you’d have to wait for Firefly to update again and then disconnect iTunes (or whatever client) before you’d see the new playlist(s). Somehow I doubt that Apple will ever modify the DAAP protocol to support remote changes because it wouldn’t make sense for you to change someone elses shared iTunes library.