FLAC tag support is now in extras-devel (tracker-extractor-flac 0.1-1)!

I have installed it, and it seems to work perfectly for the small ~200 track collection I have on my N900.

Anyone who wants it BADLY and will risk devel, note you currently may need to install it from the command line as its not been autheticated yet so doesn't show up in the app manager. You have to "apt-get install tracker-extractor-flac" and tell it you are fine with the package being unverified.
If you do not know how to do this then please do not attempt it, you could brick your device!

Obviously, not for the faint of heart but if you are using extra-devel you are obviously pretty brave anyway.