We did Owl's Head as an overnight last summer. Our site was a small open spot that we reached by crossing the river right across from the cairn at the slide. (I believe there was also a sign on the cairn-side of the river indicating no camping, which is where we crossed.) It was a legal site and roomy enough for a small tent -- you wouldn't be able to fit a lot of people there though.
There are lots of illegal campsites along the way -- but don't be tempted as the rangers said they go through and tell people to leave fairly regularly. A lot of the illegal sites are posted for no camping.
- Ivy