pylon:
You cannot forward a port to an interface but only to another IP. If you know the IP address of the Raspberry Pi connected to the Ethernet interface you can forward any port to it.You can do that even with the web interface (luci), just choose Firewall from the menu Network and click on the tab "Port Forwards".
Sorry, I should have specified that. I am trying to port forward anything recieved on 192.168.1.111:12345 (The Yun shield's IP address on my local home network) to 192.168.241.2 (The Raspberry PI's address on the ethernet interface, with no port in particular). I have tried to use the luci interface to do so, but all I get is a "page cannot be reached" when I try to connect via my browser on my computer