Looks like Wii U GamePads are region-locked as well
Nintendo implemented a region-lock on the Wii U console. As a result, titles purchased in North America won’t work on a system in other countries (and the same goes for other territories).
The region-lock seems to goes beyond this. It appears that GamePad controllers are also locked to their country of origin.
It’s worth noting that all GamePads contain region codes in the model number. That should be the key when it comes to determining if a controller will work in a certain territory.