I think you can do it when you hit a waypoint and that might be more useful. Still, for a trigger, you could group the trigger with the chopper and then in the trigger:
OnAct: nil = [] spawn { { if (((assignedVehicleRole _x) select 0) == "Cargo" ) then { _x action ["EJECT", vehicle _x]; sleep 0.5 }; } forEach (crew (thisList select 0)); };
(I assumed you wanted just the passengers for a paradrop, not the pilot/gunners).
In a waypoint, you would use:
OnAct: nil = [] spawn { { if (((assignedVehicleRole _x) select 0) == "Cargo" ) then { _x action ["EJECT", vehicle _x]; sleep 0.5 }; } forEach (crew (vehicle this));};