Hi Fred,
Last winter hubby and I entered Wivenhoe Dam here in Queensland- a gated park- for some serious observing. We had only just (within mins) unpacked and set up everything- tables, chairs, scope etc. Then the security guard came to us and said they were closing up and said to us politely that we'd needed to leave. He did feel quite bad for us that we'd gone to all the trouble, but he said there was nothing he could do about it as it was private property. While we were packing up, he looked like he was doing quite a thorough job driving around making sure no one was around. When we left, we could see him securing the main gate. I'm guessing that If there was an unattended car in the park, they would hunt you down until they found you.
Now I'm picturing you parking your car outside and carrying all your equipment in over several back and forward trips, snacks to last you the whole night long, and a very thick scrub of bush for you to hide yourself and your equipment.
Thrill seeker you.
Perhaps you could make a request, but even so, I don't see that you'd be able to get back out during the night once those gates are locked.