Just my two penny's worth.....
I practically live at a gliding airfield...and very regularly see aircraft fly overhead at heights which our winch cables can quite easily reach. I have also had to take action to get out of the way of a group of Spitfires and a Hurricane as they came home from an airshow last year...transiting right over the airfield seeming oblivious to the thirty or forty gliders in the area.
However - I have often wondered why gliders do not have nav lights or something similar. Everyone has probably seen the little flashing red lights that people put on their bicycles...they are very small and battery powered...and also very bright. I wonder why couldn't these be 'stuck' on the end of each wing of a glider. They are lightweight and very easy to maintain, plus cheap...and would make gliders a hell of a lot easier to see.
Just my humble opinion...trust me I know what its like to get too close to a glider whilst flying a powered aircraft!!!
Sam