I think in the case of a under body panel the mesh works fine because the airflow is parallel to the surface. However if the mesh is perpendicular to the air flow, like in a grill block, I would expect the air to go right through like a window screen. It might be possible with a fine enough mesh to create a pressure barrier that blocks flow at high speeds.
The effect of the mesh can be tested by putting an air flow meter at one end of a tube and the mesh at the other. Then mount it on the outside of your car. You could then determine what size mesh you would need and at what speed the blocking occurs if it does.
