[Frank Lee]

FWIW, I don't think that strategy will do as much good on a wagonback as it does on the Pinto hatchback (Edit: and Beetle). My reasoning is, if the Pinto spoiler really does work and promote attached airflow all down the hatch, it would successfully have reduced the wake area at the rear of the car by a large amount... half? But on your wagon you have proposed extending the spoiler up to such an extent that even if you do achieve attached airflow on the top of the back window (or the mimicry thereof) it looks like it wouldn't reduce the area that forms a turbulent wake very much. Now if that spoiler went horizontally out the back for, say, 3', you might have something.