For some SpecialEffect work, I've been asked if I can find a way to get a custom light-pressure PC based controller working on Wii-U to play Mario Kart 8 and Super Smash Bros Brawl. Looks like the XCM Cross fight converter might do the trick if it will daisy chain to a Cronus Max / Titan One set-up.
This potentially means that a wide range of PC, Playstation 2, 3 and Xbox 360 controllers may end up compatible with the Wii-U in this way. This said, I've never actually played with a Wii U properly, but I'm going to be learning very soon.
The adapter is said to convert a wired Xbox 360 joypad (or equivalent) to act like a Wii U Pro Controller (the Wii joypad that looks like an Xbox 360 joypad). There's a list of supposedly compatible games on Wikipedia which looks like a pretty decent batch of games (plus its Wii compatible to some extent). My only worry is how compatible it will be, and how reliant on the Wii U Game Pad monstrosity. Fingers crossed as if it's possible, all the current range of game consoles and the previous will have some nice access solutions off the shelf.
[UPDATE: 4th Jan 2015] Good and bad news. Good news is that this works with the official Xbox 360 joypad, a Razer ONZA and quite a few Playstation 2 controllers through the PS2 to Xbox 360 adapter I tried. Bad news is that the Cronus Max, Titan One and XIM4 do not work.