I can tell you for a fact that the right developer could solve this problem rather quickly and this is how I know. First off I got a moment after being with windows mobile phones for years. They have been doing it for years and well. I mean it worked every time. I never had a problem not once with three different phones and countless (probably 10) headsets. I could not believe when I got the moment and bluetooth support was so shoddy. I mean bluetooth has been out for ever and the phone couldnt even transfer files. Secondly, on more than one occasion my moment has messed up and stuck after a call and ALL of the system audio WAS routed through my mono bluetooth headset (Moto H710). It has actually worked, just did not stay that way, so it is strictly a programming issue. I have now purchased a Jabbra headset that supports A2DP and it will work but still plays audio through the speaker as well. Why is this so hard? Because it's not important to Google. I have created instances in Google codesite here:
Issue 7906 - android - Audio routing to mono bluetooth headsets - Project Hosting on Google Code and still no love. It's stupid!