For ACAS equipment, such as the TAS600, to work it's only necessary for mode C transponders to be correctly calibrated. The common reference pressure setting is then 1013mb, so vertical distances between aircraft can be easily calculated relative to each other, rather than using height above MSL.
Yes, correct; I was referring to present or future GPS-based solution. The existing TCAS systems all work on Mode C returns, which in turn rely on the Mode C transponder having a working altitude encoder.
It would suprise me if a FL-based TCAS system was proposed even for the very low end of GA, given how many of those people are dead against installing any electrical equipment, on the grounds that they don't have power available.
GPS altitude seems to be a really easy way to deal with this, especially with EGNOS coming in soon, offering vertical accuracy of the order of a few feet or better. So many altitude encoders are 100-200ft off...
With an altitude encoder you have no way of knowing if it is out of calibration, and if the aircraft in question does not have a transponder then the pilot may never realise it is out, whereas a GPS can easily report if there is something wrong with the accuracy of its 3D fix.