Well the State Beach Campground has a little cottage by the cliffs in the campground, and that's where a lot of weddings and events go on. So maybe try contacting that specific campground, and ask to speak to someone who is in charge of the events at the cottage.
They also have a group campground area that accommodates I believe 60 people. I would follow Shaun's advice, as it would have been the same advice I would have given you. Have a wonderful day and hope you find what you're looking for to accommodate your wedding. Also if you look at Google Earth or Google maps there are lots of hidden gems around San Clemente and Capistrano Beach area that are public use areas and people have impromptu small weddings and anniversary parties. Good luck and congratulations.
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