I too have thought about this - just to be sure we are talkign about the same thing, you want the N800 to be able to control a mobile phone (say), and for its microphone and speakers to be used as a handsfree by the phone?

Emulating the button is easy. Create an rfcomm listener which simply sends AT sequence.

Getting the sound to work might be a bit more tricky, need something to make the tablet emulate the SCO audio device in the right way, all the linux stuff I've seen so far suggest it's possible but stuff appears to have been written to access a bluetooth headset from linux.