UPDATE - The Japanese message above reads, "The update cannot be completed with an overseas Wii U GamePad." In other words, it seems that you can use an imported GamePad to play games, you simply can't apply firmware updates without the original.
Read more at http://www.destructoid.com/uh-what-wii-u-gamepads-are-also-region-locked-update--241255.phtml#BI594cqo50ICbhvq.99
This might be a step too far. Looks like you can only use Wii U GamePads if their from the same region as the Wii U itself. That means no Japanese GamePad use on your North American Wii U. I don't know how often this issue would crop up, but it still seems a bit silly to implement.