Oh, I see, yeah... I guess I wasn't thinking about it that way.
I was thinking let all the air out freely, but when it's being drawn back in, impede the flow only slightly (rather than completely).
That way, you still end up creating a negative pressure zone in the case, but slightly closer to atmospheric pressure, therefore reducing the over-rev effect. Shiit, if it could be made adjustable you could tune it on the fly!

Personally, I think if you impede the airflow on the way out you'll be robbing power from the engine.