IIRC, consumer wifi routers supposedly support a protocol that allows a computer to request a temporary port forward and release it when its finished using it.
Pretty much every torrent software supports UPnP/NAT-PMP today. The problem was UPnP came a bit late - the spec was published only in 2008, and by that time everything was fire-walled off and required more-than-idiot-capability to expose.
Anyone remember what that protocol is called?