While a proper shroud should work as described above, I'm not sure how functional that "shroud" (ie house soffit) actually would be.

On the other hand, I have not specifically tested myself but once you get up to a certain speed, the air forced through the rad because of the airflow against the rad, should be greater then what the fan/fans can pull through. At this point, that shroud should be blocking airflow. This is why fans are really only needed at low speeds/stop & go traffic.

Air will always take the path of least resistance, so if the path to the rad is not sealed from the front of the car, to the rad, the airflow will much rather go around, vs through the rad.