"The Sno-Parks site quite clearly prohibits camping or overnighting of any kind."
You are reading too much into that. They don't want you camping _right_there_.
You ask about Southern California. You might want to define that. Maybe as far north as Kings Canyon? Sequoia has all kinds of places.
Why do you need a campground? I can't imagine what I would do with a campground.
If you need a road and a trailhead, then maybe this isn't for you, and you might want to check out the organized trips that I mentioned earlier. Some REI stores even organize these trips. I've even gone on winter trips organized at state parks, but you'll probably have to pay a park entrance fee.
This may be a little too far north for you, but Yosemite has the Badger Pass Ski Area with a large parking lot for skiers. If you want to park overnight, you go to the ranger station there and tell them that you want to camp out at Dewey Point. The rangers will issue you an overnight parking pass, write your name into a register, and then you are on your way. Some people get about 1.5 mile in that direction, find a place, and spend the night. Some people go 4 miles in that direction, to the point, and spend the night. If one of the parks closer to where you are have a situation like that, it's good.