i think the problem may come down to the ai in opf. if you tell a loon to fire at a chopper, the loon will want an anti-aircraft weapon. similarly, tell a loon to attack a tank without an anti-tank weapon and he'll report 'negative'.
dunno how you might get round this...
perhaps name one of the crew in the chopper, then put
shooter_name dofire vehicle crew_name
when you want the shooter to open fire. no idea if that will work or not.