So, I finally figured this out. It's very flawed though...

NOTE: I've only tested with my own compiled libao and mpd which is newer than the one on the garage page and heavily optimised compared to that one so speed may vary.

Unfortunately, mpd's native alsa output does not work for this; so I made a quick workaround with libao..

1. Use johnx's a2dp-support to make the .asound etc.
2. Edit vi /etc/libao.conf and make the device "alsa"
3. Edit ~/.asoundrc and duplicate already present lines in the file into a new section.
4. Call it pcm.!default
5. Edit /etc/mpd.conf and enable the alsa output and disable the ao output.
6. Restart mpd.
7. Someone write better instructions ;P

Bugs:
- I'm hearing a ticking sound. Using alsa with speaker does same. using alsa09 in libao plays back like esd; fine but does not work with a bt headset.
- mpd's volume control does not work with a bt headset but it's possible to modify the mpd conf to do so.