The advantage I see of requiring users to host at github in a separate repo vs allowing arbitrary hosting is that you do not have to worry about spam links nearly as much. Just ask user for the repo name and that's it. The script that processes the requests can then verify that the provided name is a valid github repo and that it meets the standard guidelines and pull the readme in automatically as well as the link.
So has anyone started on any of this? is there some community controlled github that we could start with? I'd be interested in contributing some effort to this.