... or place several snipers on the map with deleteVehicle this in their init line. Then they will only be on the map if the player has selected one of them in the MP selection screen (because only AI can be deleted, it never happens to players).
The downside to this method is that there may be more then one sniper on the map if there is more than one east player. You may, however, prevent this by making an endLoose trigger around the staring zone (Montignac) that counts the number of east units present within this zone. If the result is larger than one, the trigger fires. I saw this method in an assassin mission once. And the mission maker was kind enough to point out that "only one player may join group Alpha Red" in the selection screen info box. I don't remember the syntax for the trigger though.