It might be possible. You'd have to experiment to figure it out. I have bluetooth on my iPaq and it mentions a COM port Serial profile, and since I can connect to the internet over my cellphone and to my desktop that has nothing but the TCP/IP protocol installed I think that it might be a possibility.
Let me put it this way... if you can figure out what the IP of the devices are when their connected via bluetooth then yes, it is feasible to play multiplayer.