The Golden Shiatsu!
So take a couple of the guys that just did a really hard stunt and tell them they need a reward for what they did, so you take them to a Shiatsu Massage place. when they go in have some hot girl invite them in and get them all relaxed and are laying face down on the table, get the girl to put a nice warm dark colored towel over there heads to help them relax, as well so they can not see any shadows or sudden movements around them.
Ok for this stunt i think your gonna have to get a girl that can sell her act good, and is not squeamish to touch another persons piss. If it helps I'm sure she could wear gloves.
Get her to put an Unmarked bottle of yellow liquid in the microwave or boiler or what ever they use to warm up the massage oils with, to make it seem like she is actually warming up oils. During this time a couple of the Jackass crew members come out quietly with a stool of some sort. They will set it up beside the table where there guy are laying down get up on the stools and prepare to piss.then the girl will get the warm yellow liquid and bring it back beside the table(s) then she will say ( ok I'm going to squirt some of this oil on you for the massage, and to warn you it might smell a bit musky, but it will be good for your skin. then the guys will start pissing all over them...LMAO! when they are done she with her gloves will start rubbing the piss into there skin.
If they still don't realize its piss try to get a couple more guys to come in and piss for added oil lol
1. they guys shouldn't shake there dicks after peeing nor should they do those last few squirts of piss, it might let them know its some1 pissing.
2. Maybe the guys could crouch over them some how.