Back to the topic....
I fly out of a place literally a few miles away from Lasham. I started my flying career on SLMG flyiing next to Lasham..and yes it is hard to spot gliders! Our powered gliders have the dayglo-orange 'conspicuity' patches on them and i have to say, it dos help from certain angles, but in some light and viewed from some angles you just cannot see them clearly or at all until they are close.
With putting transponders in gliders...can you imagine what farnborough radar screen would look like during championship week at Lasham??! Yes, it is a good idea and we have transponders in ours...but the conditions in which transponders in gliders were used would have to be thought about...
Powered a/c are normally advised of heavy gliding sites by ATC and the answer at the end of the day is - It is your responsibilty as captain/pax to keep a good look out going at all times.