Pines,
This site gets crazy busy in summer so the Rangers do check usually before 9pm so you could take a chance and turn up late.
Most people are not aware that most of the land around 120, Groveland, and Hetch Hetchy is National Forest land so you can camp there as long as you follow the USFS dispersed camping rules including obtaining a campfire permit so you can use a stove, I'm just not sure what conditions are like since the big fire there a few years ago.
Other options are to get there early and try and grab a site at Camp 4 or head to the campground office in the corner of the Curry Village parking lot and get you name down on the wait list. They usually start allocating the no-show campsites after 3pm. Consider the first come first served campgrounds such as Bridal Vale creek, Tamarack Flat, You will improve your chances if you can get there Wednesday or Thursday rather than Friday.
Good Luck