Reviving an old thread with a new solution. Hope this helps someone.
I found that Media Button Router did the trick, but it would prevent bluetooth-streamed music from resuming after a call or some other interruption. The app would grab the "play" sent from the stereo when the call ended.
My fix was to use Tasker (tasker.dinglisch.net). I set up a profile that triggers when connected to the bluetooth in my car (state >> net >> bluetooth connected). The action is "media >> media button events >> grab". This makes Tasker take-over bluetooth keypress inputs.
If I want to listen to music from my phone, whatever media application I launch takes over bluetooth control from Tasker. That means that when a call comes in, music will pause, then resume after the call ends.