I got it working using a cab someone created over at XDA. It does disable using voice command by simply pressing the button on your headset. However, you can still press the voice command button on the side and either speak into the phone or the headset.
I am not sure what other side effects it has, but it does work simply by installing the cab. I would suggest a backup first if you haven't already. Also, the sound redirect only works if you are not in silent/vibrate mode (you must see the speaker icon).
Bluetooth Mono Headset + Redirect sound to it (cab) - xda-developers