I only have a workaround. If the helicopter is always roughly in the same place then you could do the following.
1. Remove ammo from the harrier
2. Make a trigger somewhere i a good place near the helicopter
3. Make the trigger to be activated by the harrier
4. (name your helicopter helo1). In the trigger type in (in the on act section): "helo1 setdamage 1" without the quotes (0 is fully functional, 1 is completly destroyed).
If you are good at spawning things then you could always spawn a missile that impacts the chopper but i don't know how to do that.
I think the ai engagement is pretty hardcoded in to the engine. As i said, this doesn't really solve your problem, just allows you to get around it.