Do you share more than the DB via the NAS, for example playlists? I had trouble, when I tried it (See above.)
My NAS does MySQL, torrents, NFS file sharing, SMB file sharing and Irssi.
The database is accessed over standard TCP, XBMC logs into the sql server for the info.
What problems did you run into what trying to share playlists?
Might be worth starting a new thread as this one has the potential to be hijacked.