This may be tangential to the topic, but it could perhaps be of use.
For all the strengths of mpd, there is still a lot to desire from a library-keeping viewpoint. One of my main gripes with mpd is the inability to handle files that lack correct track numbers in the tag. (RuneAudio circumvents this to a certain degree.)
All in all, the library needs to be kept tidy, and mpd is not the tool for it.
What I've done (also on a Mac, and I have not found a single tag editor that even works), is to use Subsonic (
http://subsonic.org). It is a music player and a streamer, and it gives me access to all my music wherever I am through the web and app.
Subsonic's way of organizing the music library is by using folders, just like RuneAudio. It will find artwork for you, and the web interface ID3 tag editor is excellent. It will also give you different views, like recently added / most played etc, and I was using it to create playlists before RuneAudio 0.3. I always double-check all tags in Subsonic before updating the mpd database.
I realize that this might not be the solution you're looking for. It's my way of organizing my library, and I hope it can be of some help.
(I can't say much about iTunes' abilities, as it doesn't support flac.)
(Edit: A couple of minor typos, message was sent from my phone.)