I've wanted to camp at Bahia Honda for years. I would look at the available sites 11.1 months in advance to see if I could get in when the booking window opened up but there were NEVER any sites available for the weekends. So I started to study the situation.
As someone else mentioned, everyone would book a week or two before the 11 month window and simply book a longer stay to include the weekend they wanted. Then they would cancel the unused days.
This is the ONLY way to get a weekend reservation in Bahia Honda or Curry Hammock. It's not about ethics or morality or change fees or cancellation policies at all. If you want to book the weekend in advance, this is what you MUST do. And you'd better be on your computer at exactly 8:00 when you try it. The competition is fierce.
But, as the expression goes, forewarned, is forearmed. You now know what you need to do. Good luck.