A report on the Japanese site Rocket News reports that there is a catch for bringing a Wii U controller to another region for use. For example, using a Wii U controller from the West in Japan may not "region lock it" (bar you from using it) but it will not allow you to apply any firmware updates. Earlier reports suggested that the controller could not be used with a console that might be from a different region. A translation by this NeoGAF thread of an error message clarifies the problem a little bit:
"The update cannot be completed with an overseas Wii U GamePad," reads the error message.
This simply means that your foreign controller won't be able to take a firmware update in another region. Apparently Wii U controllers have their own special firmware separate from the system's firmware updates. Ultimately this does not matter for now because none of the games available right now in any supported regions are capable of supporting more than one Wii U controller. If that changes then this story will have more relevance, but it's still good to know...