Its sounds ok.
Maybe the ip that you put is the one that DreamPi uses and not the Dreamcast one.
To be easy, make a broadcast of your network and see the DreamPi IP, ok. Discard this.
Now, play some game and make another broadcast to your network, now you see a new ip of DreamPi when the DC is really running online, thats is really the IP that need the port forward.
I hope that this is what fails in your planet ring try.