You'll need to find a celebrant who will marry you in a cemetary as well as get the cemetary dudes to agree. I've organised security for a cemetary so I have some idea of the kind of stuff that idiots get up to at night and how much work is involved in trying to clean up the mess. I think it is very unlikely that a cemetary would let you in at night or for a wedding because their primary concern is with protecting the dignity of the grave.

We had a religious ceremony on a differant day to the reception and invited heaps less people. Then we went to the registary office to make it legal and get the paperwork almost 5 years later (about 2 weeks ago lol).