Does the glider site have an ATZ? Is there a 'cable launch hazard altitude' marked on CAA 1/2 and 1/4 mils? (NB Jeppesen charts do NOT show this as far as I'm aware).
Anyone who does this is asking for trouble in my opinion; I've warned repeatedly on these forums about cable launch hazards but people are seemingly oblivious of the damage a 1/2 inch steel cable can do to a metal or wooden wing. When winch driving many years ago at Halton, I've 'chopped' the launch more than once when I saw a powered aircraft clearly flying towards the cable below the glider.
As for flying in the vicinity, well I've experienced a mid-air between a glider and powered aircraft inside my ATZ and I don't want it to happen again..