There doesn't seem to be a safe mode/maintenance mode like the Wii or 3DS, at least not one that has been discovered.I booted up my Wii U console and it downloaded the update due to a router reset that unblocked all the connections to nintendo's servers.
I haven't updated it, but if I turn it on it just says "Wii U will now update" with "OK" as the only option. Is there a safe mode I can boot into (like the 3ds) so I can get out of this lock?
I believe if you had standby functions on, then it might install the update while the console is off, so consider unplugging it while a red light is showing.