Ok, front windshield cowl is a good idea. I forgot about that. My 1940 Chevy got its air that way. I don't know why I thought things might have changed.
As for the rear heater/ac, this has been suggested several times. I am not concerned with that air, since it is recycled inside air. The problem, and topic of the thread, is how to filter the new air coming in. Doesn't anybody else notice they Expeditions are super dusty inside after driving off the pavement? Thats my #1 beef with the Expe, cleaning out 1/4 inch of dust from everywhere, after a nice drive.