Why build it into Wi-Fi? I have thought for many years that you should build such a service say local.wifi.com that someone accessing a wifi network could use. While I'm not sure of the technical details of automatically capturing the AP - you could allow everybody to comment - like digital graffiti or digital breadcrumbs, and provide other services as well (e.g., reviews of restaurants local to that AP). Centralized, this information could then be aggregated for others to access who aren't on wi-fi.