Um, not altogether sure off the top of my head, but looking at the a2dp script (in /usr/bin I imagine) which johnx's package installed should give you some clues. Look at the section which handles the "runfirst" switch and see what it changes.

Simply running "a2dp toggle" from the command line and trying the headset (if it doesn't work try toggling again) should get you up and running, though you may need to re-pair to get it working (I have to do that when developing, seems to get its knickers in a twist and not let you know why).