The suggested method works in both MP and SP. As far as I know it can be used for human players as well (havent tried since OFP though). The unit will keep it's scripted FSM behaviour after the change, so a civilian unit grouped to a (non existing) East guy will take cover and hide like a civillian, even if you give him a rifle.
You may also change a units loyalty by editing it's side in the mission.sqm (opens with any text editor). No need to group the unit to a non exsting leader if you chose this method.