Interesting to do this in hardware, as it will work for all devices on your network: also tablets and so on. However, I wonder how many people have separate modems and routers: Most providers (here at least) have modem/wifi AP combo's, which makes it impossible to put this in between.

Also, I wonder if some sites will stop working: e.g. I can imagine a video with a pre-roll just never starting, when the pre-roll can't be loaded. And it being hardware, this will be harder to skip.