Yes thats the only 'permited site'..
you wouldn't really need to wait till 9 for the ranger to open the gate at castle rock because you can't park there overnight anyway...
here's what we did for parking that I mentioned in another thread....
at the top we parked in this turnabout.
(click street view.. we parked next to the callbox)
We drove around a bit and there were no 'no parking' signs anywhere on that side so we went for it.
At the bottom from what i remember there didn't use to be 'no overnight' parking signs next to the main gate but there are now so we drove up and ways and found another little turnout that didn't have any signs.
(click street view.. right next to the log)