I've seen ways of using a bluetooth headset as an audio device for linux, either using the newer blue to register as a alsa sound device, or older method using btsco to create a sound device. I've had the latter working, but not the former.

For using the N800 as a bluetooth headset (or loudspeaker) I would guess you could adapt the btsco code to create the relevant sco service, you'd also have to change the device class so that it was recognised as a headset. Emulating a button is easy, create an rfcomm talker/listener which sent the AT code.