I can only speak with anonymity in mind, since this would mean you run a "vpn" service (having a "quad-vpn", but actually one that makes sence in that its not just 4 different servers of one company). Having an access line would either mean you'd need to identify yourself (and of course it takes way more efford than just downloading the browser and getting right to it) or you'd still have people going to access lines and afterwards abusing the exit node.I'm not sure how you would implement port knocking and iptables tbh. Still, I think it's technically a good idea, I personally just wouldn't know how to implement it correctly.
Best, shruub