Port swap across tunnel

[ZAero]

My HE IPv6 tunnel is running through an Arris NVG468MQ router which is configured for 6rd and static LAN.  On two of the machines behind the router, I'm running an app that monitors port 8769. This app has markedly higher throughput when that port is forwarded in the router.  This is true whether the app is configured for IPv4 or the IPv6 tunnel.  While a different port can be selected in the app, the public facing port number cannot be changed.  The router does not allow forwarding the same port to two machines.
    While I'm curious why the port forwarding would affect the IPv6 tunnel, my question is this:  Is there a way to map a port at the WAN end of the IPv6 tunnel to a different port at the local machine?