I'm gonna have to try this, but I think the mic on a BT headset might work if you have the BT headset connected. I know that when I walk to my car (listening to music via HSP), and plug it into my stereo, the sound switches; but since I've got a 3-pole (TRS) connector instead of 4-pole (TRRS), it'll get the mic signal elsewhere. Could be the N800's mic, but I'd expect the HSP mic. And I know the HSP button works.

Edit: just tested; uses the N800's mic, which may be suboptimal. So if you've got:

• The N800 placed where the mic can pick up your voice
• A (stereo-output, 3-conductor) A2DP dongle
• Some A2DP device to play that
• And a BT HSP headset tie-wrapped to the steering wheel

then you're good to go.