believe it or not this is what some NASCAR teams do (or used to do). Aft of the bonnet and forward of the windscreen is a high pressure area. Now, POD filters, unless enclosed in a box, are most certainly illegal.
Cutting the firewall, another thing I wouldnt be jumping into. Cutting, beating, grinding etc etc of the firewall can result in your car being black listed (for suspected VIN tampering) which in turn means you'll never register that car again, ever. You can risk it and providing your cars never stolen etc etc you should be right.
Some cars (can't think of any off the top of my head) Have plates you can take off to clean out the vents and Ive seen at least one rotary guy doing the exact thing your suggesting here :doublebird: