piss in a glass is a bit of a rugby club cliché ;)
I'd find out who the ring leader is, hopefully he'll have a significant other. Next night out, get some fit bird to flirt with him and discretely capture the scene on your camera. email it anonymously to his girlfriend. buy a cheap phone, send the photos to it. drop it off with his girlfriend, pretend he lost it. she'll want to know why he has got a second phone, hopefully curiosity will get the better of her. get another sim card for your phone, send a few suggestive texts (I had a lot of fun on friday, can't wait to see you again) before he gets home. you could even get random women to phone his home number.
hopefully he'll end up in a fair bit of shit, if he figures it out, he'll still have a hard time explaining to his girlfriend what he did to piss you off.