Here's one thread I saw-
Reverse NACA duct [Archive] - AviationBanter
and another-
Studying aero: NACA scoops in hood? [Archive] - MX-5 Miata Forum
there are more but it seems fairly obvious to me that NACA ducts are directional so like anything that's directional they would function best in the direction intended.
Try driving your car backwards on the interstate, or hammering a nail with the handle of the hammer, or gnipyt sdrawkcab no ruoy draobyek...you get the idea.