I'm interested that your TCAS shows the aircraft squark code, how many do this? Apart from that I think it's a silly idea (sorry), you're not surposed to use the info displayed on your TCAS to provide seperation, let alone some micky-mouse, homemade version.
Do you have much non-trasponder mandatory controled airspace in Aus? We've all but done away with it meaning no aircraft can operate in controled airspace without Mode C, unless given an exemption. Of course if the aircraft is operating below controled airspace you can be fairly happy it's nowhere near you anyway.