If the area is posted dawn to dusk, I would take that to mean the parking lot also and wouldn't stay there out of respect. If it isn't posted and wasn't where you would have to worry about the local kids coming out at night, especially Friday/Saturday night I would consider it.
IF there is a nearby church I would try to ask permission, as with any place with someone nearby.
But yes, I've seen some very small quaint cemeteries in areas that I would be consider camping at and don't think I would give those a second thought.