Hi Paul,
Funny you should mention the idea. I was thinking along the very same lines myself. Im in the throws of constructing a cold air box for my 172 to house a K&N 57i. Originally I was planning to have two 50mm air pipes going into the box, one that was "force" fed from the front grill and another that takes air from internally below the headlight area. I was planning to put an in-line fan in the non-force fed pipe that was running all the time, so when you stop or are in slow traffic, cooler air is still being fed to the filter. I found a 12V 48mm inline fan in RS Components but they were looking for £70! So, kindo went off the idea. Im now running a 75mm Pipercross ramair pipe kit between the box and a cutout on the front grill (when DTs eventually get round to sending it that is!!)
Neil