Is how can I enable my wifi router to accept Bitcoin for adhoc/ anonymous usage?
There is a project called wifiportal21 that uses the 21 Bitcoin computer in conjunction with an OpenWRT router running the wifidog captive portal software.  This is still in beta at this time, but seems to suit your application.

It is also worth noting that OpenWRT does have the ability to create a separate network that cannot pass traffic to the main network, and works with RADIUS authentication.