I would probably leave off the full moon part, although I think a neat idea few would probably follow that. Or if you want only place the cache as available during a full moon. However before asking that they visit at night, I'd get in touch with some local officials or caretakers of the cemetary and ask, just to be on the safe side. Here there are hours posted on only one of the cemetaries, the city run cemetary. However I have been in the cemetary at night, going just before sunset once to show my mother a stone, and left after the posted closing sign, and during that time I saw a lot of people coming in and leaving the cemetary. I am not sure if it is illegal or not, but be on the safe side and ask. :-)