Yes, I know you can't fly them currently--and I'm not suggesting mad code edits. But I also know that the (rather creative) Harrier flight model is practically halfway there: ever flown a Harrier with full-flaps at low throttle and altitude?
I profoundly doubt you could replace actual F4 helicopters with something flyable. Instead I'm thinking you'd create a new "plane" and give it a helicopter flight model, 3D model, and weapon loadout.
But I'm a newbie (especially to flight-model editting). So I'm interested in some expert thoughts!
-Cal
P.S. Why? Well, I happen to love hopping around from flight to flight in a campaign (in AF anyway)--and I think adding helicopters to that would be FUN--simple as that! No one is going to build a new multi-aircraft Falcon, so why not give this a try?