It's nasty, but at the start of the mission you can check whether the player is in Veteran or Cadet mode: if he is in Cadet give him a hint saying what's going to happen, then end the mission. (I think the command is cadet, not sure - look it up.) Remember to say in the Overview that the mission can only be played in Vet.
If the player is part of a group he will still be able to get his loons to report their status, which inclues their position which is obviously the same as his if he is close to the loon in question. The only way you can force the player to do map work is to have him solo.