Instead of a TCAS for GA, with all the inherent inaccuracy of azimuth, how about a GPSCAS? It surely wouldn't be very hard to develop a GPS that transmitted it's position, and received data from other GPS's for conflict calculations.
The Marine world has something similar already with the Ship Identification system, although that (at the moment) is not used directly for collision avoidance, though it does assist identification of "blips" on the radar and can help avoid the oft-heard RT transmission "ship on my port side, what is your intention?"