You don't need to get the user to open up ports to build a home server appliance that seamlessly works. You just need some people with routable IPs to serve as

You don't need to get the user to open up ports to build a home server appliance that seamlessly works. You just need some people with routable IPs to serve as rendezvous points for the rest. These machines can be totally untrusted. From there you can bootstrap a peer-to-peer VPN.