Although the official policy on Hike&Bike sites does say they can turn people away if full, my experience is that they won't turn away any legitimate bicycle camper. They've opened up day use areas (i.e. picnic sites) and other parts of the park when necessary to accommodate extra arrivals with the provision that if camped in those spots you take your tent down fairly early the next morning so other people can use the area. Not sure what gap you mean in the Big Sur area. Big Sur State Park has a very nice (and large) Hike&Bike area, Andrew Molera just a couple miles to the north has walk-in campsites that are fine for cyclists, and both Kirk Creek (Nat'l Forest) and Plaskett Creek a little south of Big Sur SP have Hike&Bike sites. [On my last visit Kirk Creek didn't have potable water, but the camp host offered me some if I needed it.] So lots of options, but I'd recommend Big Sur SP as having the best facilities (camp store, gift shop, restaurant, laundromat, hot showers, assorted hiking trails).