Is how can I enable my wifi router to accept Bitcoin for adhoc/ anonymous usage? I want to find a WiFi solution where I can sell my internet service with anyone who needs to use it... ideally paying in an altcoin or Bitcoin.

Are there any router modifications, or infrastructures that support a wallet service?  (e.g. the user pays at the captive portal) and then has time or bandwith limited access to the internet?

I found this list of router firmware that may support such modding, but not sure which one is best (and most secure) for the job. 

Lacking that, perhaps I can leverage RADIUS authentication.
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.