It would help if you could somehow get a group of people who don't know each other very well. The combination of not having any connection to one another and that initial social awkwardness would somewhat discourage the attempts to fight you off while raping one of the others.
Also a good idea.
You think the mods would be okay with "ITT we discuss how to pull of a reverse gangrape"? Maybe we should call it "legal gangrape"?
Maybe the same phenomenon that makes a school bully get away with his behaviour and allow a dictator to suppress an entire population will let a reverse gangrapist get away with raping hundreds of people at the same time: all of them are afraid to be the first to attack, and they're too confused and afraid to launch a coordinated attack.