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).
That's not possible, you have to forward to a specified port. Else, how would the Yun shield find out what to do with a packet received on port 12345? Forwarding always needs a target IP and a target port. The port number may be the same but it must be predefined.
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
That sounds like a network problem. How did you configure the Yun shield? Did you set the WiFi interface to be the internet connection?