I think that glider pilots are more sinned against than sinning, but...
(1) It can't be that hard to put stobes and a power supply on a glider without damaging it's performance.
(2) Yellow and orange are also very visible but still reflect UV and thus don't damage the GRP.
(3) When you spot an aircraft that hasn't seen you (and flying microlights much of the time, I understand the problem well) it is sensible to tip your wing planform so that the other chap can see it. I do this regularly in the microlight, and don't see why a glider pilot can't rather than just ignoring the other traffic - which is legally correct but poor airmanship.
(4) It would be no trouble at-all to stick a Luneberg lense within a glider fuselage, making sure that you show up on primary returns.
G