In theory, you should be able to achieve this by using the playerNear check (which can be customised per mission, apparently):
Player Near:
DMS_playerNearRadius = 800; // How close a player has to be to a mission in order to satisfy the "playerNear" mission requirement (can be customized per mission).
I would guess that in theory, if you set the playerNear check to 10000, then players could be 10k away from a mission so could therefor arty them. On some of the statics, you could then set that to 1000 so players have to be closer. Also, just realised that your last post was a year ago :S
