They only part of the rules that I see relating to this is:
17.3 Body Structure
The chassis structure must remain intact and stock except as noted.
This is a good question to bring up, because by making a modification, you are opening yourself up to protest, and in a very strict interpretation of the rules, it's illegal. That said, the rules don't address this very well, and we technically violate this any time we put a rivet in the car. This is where asking your series director, or better yet, getting clarification here, (in advance, please) can protect you.
This is probably something we should address next year to better clarify this issue. In the meantime, make these modifications as minimal as possible to accomplish ducting, and realize that it could be taken away next year when we take a good look at it for rules purposes.
On this specific issue, what are your specific plans, Charlie - where are you ducting to, any why is it not possible to go another way (i.e. through the stock intake hole)?