We have also run into this but the c.g. has let us enter regardless. I think if they were going to charge for early check in, I would park in a prime spot near the office and while away the time until they allowed us to go to our site. Might work, might not but I would certainly try it if the c.g was not at capacity.
One time in NY state, all that was available was a non hook up site, which was ok with us. We had to wait 2 hours to go and park on it despite the fact the site was empty. We were told the extra charge for early arrival was to cover the cost of park amenities we would use. Duh - no hookups, no pool etc. - just didn't make any sense to us but we parked in their parking lot and went for a 2 hour bike ride. Just a nuisance factor in that instance.