I was just blocking these, at the router:
*nintendo.net
*wii.com
*nintendowifi.net
google.com
This would account for any changes in the subdomains. Just as long as the domains remained the same, I'd be fine. I added block to google.com so I could check from the Wii U browser if the block was still working.
I went one step further today and loaded the payload on a spare android tablet (that I already had the rxtool browser payload on). Simply install "1 click tether" (free, the default android tether may or may not work as if there is no mobile data, it tends to not allow tether, but 1 click tether always works and doesn't require rooting), install kWS (free). Copy the kexploit and loadiine over. Then I setup a connection on the Wii U to the phone (ignore the no internet errors, it still creates and saves the connection, set as default). Disable (by renaming the SID) your other connections (if needed).
When you use the Wii U loadiine, you just need to make sure your tablet is on (mine is spare, so I leave it powered off when not in use to keep the battery juiced), start 1 click tether, then start kWS server.
When you're tethered to the android, your website addresses will be:
192.168.43.1:8080/kexploit
192.168.43.1:8080/loadiine
Create bookmarks as needed.
It might just be anecdotal, but ever since I setup this android hotspot for the payload, I've never had a loading freeze or race condition error. Might be purely coincidental.
I could have easily have loaded the payload on a cubieboard webserver I have running 24/7 on my network, but it would still rely on my block on the router. In an android tether scenario, you never have to rely on an internet or DNS block.
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My only remaining issue is, now that my Wii U is connecting to a source with no internet, the Nintendo ID signing in appears for a long time before aborting with an internet error. At least I don't get the annoying "update is available" + click twice to cancel.
Guess I'd have to remove my Nintendo ID (reformat) to get rid of any attempt to connect to the internet?