What I have seen people do in the past is to check the reservation site for the campground for any available sites on a single day period...cancellations do happen.
Make a reservation for that one night....come in after lunch, check in and just go to the site...have a picnic dinner...fish a little up through the campground...and once it gets dark head up the trail to the cabins. The woods behind cabins as you drive up to Jakes Creek area are a good area to watch and not as many people. You can also walk back into the woods next to the first cabin beyond the trailhead and actually stand among the fire flies as they fire off....really neat. I think a regular campsite goes for about $20 so this is not too bad of a deal since it would allow you to drive in to the campground and stay as long as you want. These fireflies are not like their lowland cousins...they don't really get going good until dark. As good as this show is the area around CS24 is crazy with them. They are so strong up there that my son and I could see their light through the walls of our tent one year...thought someone was out with a flashlight...the whole mountainside along Rough Creek next to the camp area was just on fire with them...enjoy.