The answer is much more simple than this. You block off the roads that lead into the Studios parking plaza. You station a security vehicle and maybe a traffic controller. People aren't going to riot or try to plow through the barricade, they'll just keep driving. The influx of people at park open on New Years Eve is far greater than what DHS is seeing right now, yet they don't allow people to park super early for that. They just staff accordingly and prepare for the crowds.
It's also funny because Disney built new overpasses and engineered new traffic patterns for DHS, built a massive new entrance road for it, built a new toll plaza for it, doubled the size of the parking lot, built a new mass transit system to the park, revamped the entrance to be huge and open, built a massive new security checkpoint, AND widened the walkways inside the park itself - all for this exact reason. And now they're suddenly acting like they can't handle it unless they let people park when they want to and open early.
And now I'm hearing that the ROTR boarding passes were once again completely booked before 8:00 AM. Yes, before the advertised park opening.