I agree with all the above comments on individual state parks with H&B sites. Your 'Bicycling the Pacific Coast' book should list which parks have such sites. On at least one occasion I stopped at a state park without a H&B site that was full but they still found me a place to camp (they had a few sites closed for maintenance activities and let me use one of those). The official rules are that the park does not guarantee you a spot even when they have a H&B site, but I've never had a problem with them turning anyone away. Even if the H&B site is too crowded they try to find a spot in a dayuse picnic area as long as you agree to take down your tent in the morning.
Note that Pismo Beach has recently closed their H&B site - I suspect because of its use by a significant homeless population. That's also been an issue in the San Diego area with parks that offered H&B sites long ago but closed them.