Its been awhile since I've been up there, but we were able to camp out near the lake with no problems. (it was early spring, too cold for swimming)
Checking out http://www.nps.gov/cato/planyourvisit/campgrounds.htm
It looks like either at Owens Creek or the Adirondack shelters have potential, just need to get the permits taken care of.
Of course we could always just wander over to camp David and see if they'll let us use the lawn