Bob's idea for weekdays is a great alternative. When I set up the camping arrangements last year there was definitely a better availability of sites mid-week.
If I recall correctly, we discussed this last year and decided to accomodate those who could not take the time off mid-week. Nonetheless, if we make it a weekday event it would definitely open up the options. We'll need to look at how it would affect those limited to weekends.
I'll call the Waquoit folks and investigate.