drink a few gallons of vodka a few minutes before you go through the checkpoint. once you get past them have your friends ready with some empty pails and simply regurgitate it all out. boom
less practically, you could gain entrance into the festival and have someone else meet you along the perimeter and sneak you the booze like over the fence or however the hell you want. fly it in with a drone if need be.
fill a balloon (one of the long clown shape-making ones) with your booze of choice and suppress it until all is clear
don't brink in booze and instead bring in LSD