I don't, the only experience I have with showing up unannounced looking for a HB site is at China Camp, in Marin Co, but the couple running the reservations was living in an RV, seemed way laid back, and even though the area was booked pretty full, including another biker at the HB campsite,
they basically told me, camp in the HB site if he will let you, find another spot if he won't, anything without people in it right now (it was a small campsite), but either way, we will get you a nice comfortable spot.
Dude was a dick, I camped in a nice spot right next to a nice family that the couple running the campground discussed with the couple before they pulled in. I was a good 40 feet away from the family, but it ended up being a fun night, playing with the kids and drinking beer with the parents.
Moral of the story is...no red blooded American is going to put you back on the the highway when you need a place to bed down for the night. There will always be a place to pitch a tent, and some can be serendipitous experiences.