To whom it might concern; The latest release of Android (4.3) now has official support for AVRCP. This in combination with the hacked version of Spotify do present the track title on my MW600 headset, and probably on any car unit as well.
Hopefully now that it will be part of the native Android stack, and all Nexus devices, Spotify will get their head in the game and fix this. Obviously, looking at the patch it's like 10 lines of code, if not even less.
EDIT: Actually, it seems to work with the official client as well. When did this get fixed?
Re: AVRCP support to see artist and track metadata during Bluetooth playback