Just to be on the safe side, I asked the manufacturer of the glider I fly whether they were legal to fit. The answer was a firm no, they are not on the approved equipment list for the type.
I also asked whether the TRIG TT21 could be added to the approved equipment list, possibly reducing the modification cost. It is under consideration, so there is some hope there.
Six years ago the process of fitting a transponder would have been easy, and a fraction of the cost it is now under EASA (assuming all other technical problems are overcome).
An example of the dead hand of anti-safety of the current regime. There is a glider type used for training where the rear seat is suspended from an adjustable strap to change the height. In a seriously heavy landing, the strap attachment breaks, increasing the spinal injury. UK gliders had a simple mod to detach the strap and adjust the height uning energy-absorbing foam. The replacement type from the same manufacturer has the same arrangement. Under EASA now either you fly unmodified, and risk spinal injury, or pay a design authority a large sum of money to apply for a major modification. The manufacturer refuses to change the arrangement, because that would imply admission that the design is faulty, leading to legal liability. Remind me what the 'S' in EASA is supposed to stand for?