Would you consider using a multiple exposure technique that I have used many times with great success although not in the situation you describe. Because the gravestones are static you can break down the exposure required to record full information into shorter times. Each time you make an exposure as a flag is placed you will record a ghost image of the person placing it. Over a period of time you will reach a point when the correct exposure has been given to the static objects in the image but will also have a number of ghost images in the photograph. You will need to carry out some experiments as to how many multiple exposures you give and how long they should be. I would think that you will have to use an ND filter to help you give longer exposures. If you look in the Gallery section at my photograph of Rocks and Water you can see the effect of that technique on running water. If you need any advice on how to calculate the exposure please contact me direct on firstname.lastname@example.org and I'll send you a copy of a section on multiple exposures from a book that I've just had published. I'd include it here but I think it'e perhaps a little too long for this forum.