Great idea. Was looking for something like this to show some prototype. Was wondering is it necessary to be restricted only to rails app? For ex., I can host a wiki and try to get static address with your service. Might want to check dyndsn sort of services.
I like the idea. I think key to success of this idea - getting people already know the subject to participate in the service. Their input might be more valuable. So it also brings down to point to ranking various lists available on the same topic. Digg/reddit like voting might be useful here and in turn might improve engagement also.
* As far as I understand chargify is not a payment gateway. Correct me if I am wrong.