If you set your network to open, you should check to see what the devices are that are connected to it, force people to use an email address and their name to use. Absolutely nothing would be preventing them from not using their real email address and name, but still. And make the default home page they get once they put in an email address jwfacts.
That way you're not hijacking DNS, you're simply setting a default homepage for people who choose to use your service when they click "I agree", kind of how like Marriott always takes you to their homepage first when you use the hotel Wifi.