I think I've got it!
so, I've been trying to do this exact same thing for about a week now, and I think I've got it. with the help of some random people on the internet.
it seems this is actually a two part problem.
1) get the device to recognize the bluetooth microphone (from a headset or other bluetooth wireless device).
2) get the audio into a video recording.
it seems that android and an OS is not readily prepared to do either of these, and the built in video recorder cannot use audio from a source other than the devices built-in microphone.....so....we have to improvise a little.
First, we will need to get android to play nice with the bluetooth device. start by installing a free little app called "bt mono" you can find it on the google play store.
once installed you will need to run this any time you want to use your bluetooth device in a way other than as a phone.
on a side note, this works great for listening to music via a bluetooth headset.
second you will need a video recorder that can take the audio input that bt mono gives you. so, run back over to the google play store and get "lgCamera"
the only thing you have to do now is tell lgCamera that you want to use the "microphone" as the audio source.
thats it. you now have a video recorder on your phone/tablet that will use "almost" any bluetooth device as the audio input.
I have this exact set up on my Verizon Samsung Galaxy Nexus with CM 10.1.0 rooted, and it works perfectly. I am not sure if this works with unrooted devices, though I don't see why it wouldn't. hope this helps anyone who's been dredging the depths of the interwebs!
very special thanks to david hadaller and to Warren Barton for the breakdown and links!