Since this thread was very helpful for me, I've taken the liberty to update the code provided by AZ to use mission event handlers instead.
Init of the helicopter involved (adjust vehicle_variable_name as you would):
1 = addMissionEventHandler ["EachFrame", {
player action ["lightOn", vehicle_variable_name];
}];
this addEventHandler ["Killed", {
removeMissionEventHandler ["EachFrame", 1];
}];
This will result in affected AI-controlled helicopters keeping their spotlights on persistently. Most likely works with land vehicles' headlights as well, but I've tested it with Prairie Fire UH-1s.
