interesting, first, as soon as potential thieves learn that the mowers have security features that make them very difficult to re-use, they will not be much of a target for theft. They beep if picked up and can't be re-started without a PIN or w'ever. Stealing metal urns/flower vases is probably easier - particularly if the robot mowers are on a timer to run at night.
The new iRobot Terra could be a good item since it doesn't need buried cables. I dunno if they can be deployed as a 'team' - so, a large cemetery may be too much area for one mower?
You should contact iRobot - they may be interested in using your situation for a 'test bed' .