Well, I had no time to check trough your question at home, but I just droped the script to my memorystick, and now I see, it's just remove the default parachute.
;// wait that the player drive the normal parachute
@ (typeOf (vehicle _HALOunit)) in ["ParachuteWest","ParachuteEast","ParachuteG","ParachuteC","Parachute"]
;// Delete the default parachute
_defaultparachute = vehicle _HALOunit
deleteVehicle _defaultparachute
But in this script: "ca\air2\halo\data\Scripts\HALO_AI.sqs" you can find how to create and equip the player with the steerable para. Good luck!