Could a remote mount transponder be a technological solution to the problem in the lightest powered aircraft? There should be space for an on/off switch on any panel, which would suffice if the unit only squawked a fixed code. That code should be reserved for such devices, so that ATC would know their limitations. The market for such a transponder must be larger than that for the GTX330, so it shouldn't be too hard to persuade a manufacturer to make them, and the extra simplicity should reduce cost and weight.
My guesstimate is that the problem is not in the physical size of the unit, but in the power consumption and hence the size of the battery in a non-electric aircraft.
Is there anybody who has more knowledge of electronics than me, who can do a few calculations based on power output, transmission losses etc. and come up with the size of a battery that can support a mode S transponder for a reasonable flight time (say eight hours - I understand that for a Silver C in gliding you have to do a five-hour flight)