As I see it there are 3 problems with fitting transponders in gliders.
1. What do you power it off, gliders on a cross country are typically in the air for 4-5 hours and 8-10 hours is not that unusual.
2. How will the receiving stations handle a gaggle of 50 or more gliders. A typical TCAS can only cope with about 30 contacts.
3. As we generally operate outside controlled airspace we are not in contact with ATC how do we get warned of the danger.
I belive that the answer lies in a low power version of ADS-B or FLARM type of device.