1) dig a grave
2) place a corpse dumping zone over the headstone
3) bury a body in the grave
4) remove the dumping zone
The body will become visible to the player. I have not confirmed if it is visible to other characters (for the "saw corpse" effect). If not, then it's just a graphical problem.
Thanks for reporting.
http://ludeon.com/forums/index.php?topic=1744.msg17142#msg17142