Use the Switch type of trigger - when these are synchronized with a units waypoints they indicate that the unit should move on to the NEXT waypoint instead.
Activate WaypointRepeat the following for each waypoint that you want the helo to move towards without actually reaching it:
place a trigger close to the waypoint in question: dimensions 150 x 150 minimum, type Switch.
press F2 and drag a line from the helo to the trigger.
(Check the trigger's properties; it should now be activated by Vehicle, Present).
press F5 'Synchronize' - and drag a line between the trigger and waypoint.
New waypointLay the player's waypoints out going via the alternative route. There should be a Switch trigger at the third waypoint synchronized with the waypoint just prior to the objective.
Thus, when playing, the player moves to WP 3 and if the event does NOT occur the Switch trigger should trip and direct them straight to the objective.
If you need it to be more intricate, that will probably require scripting and use of
setWPPos.