The one post in that thread was correct about many California beaches "closing" at 10pm. I haven't found hours for the beaches around Isla Vista but here's some in the city of Santa Barbara:
City of Santa Barbara - Parks *&*Recreation - Parks Maps - Beach Parks
I don't think this is typically enforced (I've definitely been on the beach after 10pm and never had any problems), but I assume it will likely be enforced during Halloween weekend, and it definitely would give the cops a legitimate reason to kick people out after 10pm.
I think they try to make it as difficult as they can for out of town visitors to attend. I'm actually making the trip down (probably my one and only time as an alum) and I had my friend get a Halloween permit so she could move on campus and I could take her apartment complex spot. By around 3pm, Isla Vista will be literally at full capacity as far as parking goes (even the offroad parking spots like the end of the 6800 block of DP and Camino Majorca).