I am sure a mode C transponder wont hurt either.
The problem with that is finding the juice to run it. All the other "Gizmos" are low drain and a glider (Which might be doing a ten hour flight) can run them with a couple of 12V batteries. TXPs are much higher drain.
There are moves to develop low-drain mode S TXPs for gliders, but they are not on the market yet.