Cutting back on the throttle or applying the independent would work on the level, but IIRC the train was working hard up a 3 or 4 percent hill. Even standing still on a grade that steep, without applying the train brake and then backing up a bit would there have been enough slack to uncouple?
But, as you say: in Hollywood all things are possible - except restraint. Do you think MAYBE they stopped the camera(s), uncoupled the train, and then restarted the camera(s)? But that would be cheating, wouldn't it?