I had a guy stop by several years ago wanting to do the same thing. Instead of him modifying a Linksys WRT54GL with open source software he proposed a proxy server that injected advertisements via email and web page.
Granted he wanted several thousand for his setup and it was a tad out of our range but more importantly our customers expressed their concerns with this idea. Mostly they came in to the coffee house to relax, have a cup of Joe, and read what ever tripped their trigger. They did not want advertisements pushed on them. Some even felt it was an invasion of privacy.
5 or 6 years ago someone released a survey he did of his restaurants and cafe's with Wi-fi. He found the average customer who was using a laptop spent up to $7.00 dollars in the shop before they left. The little bit I surveyed found this to be pretty consistent for the coffee shop I work with.
So with that in mind we ditched the whole idea and have never brought it back up. We do however have kiosks that charge $1.00 for 10 minutes. They push in store advertisement both picture based and we have in the past provided video as well. We also pull and use web pages as advertisement. Its all shown as a screen saver and once they move the mouse it all disappears. This system here makes enough money to cover the cost of the equipment upgrades as well as the providing the Internet access.
I think your approach is a novel idea and defiantly has potential. But there are thousands of cafe's giving a way free wifi with no strings attached and the public around here expects that. I even have people asking me to setup a femtocell for all the major networks inside the shop. There complaining about signal quality and think we should do something for them in that aspect.
