The "GA" TCAS systems e.g. the Avidyne 600 work just fine.
They give useless readings from Mode A transponders because there is no altitude information.
But in the circuit, the information is presented OK, and is usable if everybody is transponding Mode C/S.
I've for some time been tempted to get the system but it's about £15k (plus or minus 5k depending whether G or N reg, which avionics shop you use, and whether you free issue parts bought from the USA) and the biggest problem is that existing antennae need relocating and that makes the installation into a very large job requiring the removal of the entire aircraft interior, and not many avionics shops can do a neat job of that on a TB20.