As it turns out I had a few Raspberry Pis lying around, so gave this a shot. I don't have a Wii U USB <> Network adapter, but I DO have a spare access point lying around. So I:
- Configured the AP to have an IP of 10.10.10.1, Gateway and DNS settings are empty, and hooked it up to my network
- Setup the Wii U with IP 10.10.10.15, Gateway of 10.10.10.1, and DNS settings of 10.10.10.2 and 10.10.10.3 (so, pointing to nothing)
- Connected the Pi to the network
And ... it just works, no muss, no fuss
As it uses a completely different IP range to my existing network devices, they don't interfere with each other; the Wii U can't connect to any site or IP address other than the Pi, and even if I hook my PC up to the AP all it can connect to is the Pi. I tried pinging/tracert-ing some of the Nintendo Update/Shop URLs and got nothing.
Thanks for this guide and the links - now I don't have to arse around with running servers on my main PC, or have to risk the Wii U randomly updating itself!