Yes they could
Also just realised that RUFC already hold fans back
If you're in the car park you are prevented from leaving for 10-15 minutes after the game (albeit sat in your vehicle)
It's part of the T&C's when you buy the permit so as mentioned earlier make it part of the T&C's on the away fans tickets that "RUFC & SYP reserve the right to keep you within the ground/rear car park for up to 10 minutes after the game has finished"
If they buy a ticket with those T&C's they are accepting them therefore cannot object