Though clever modders will always find a way
When Super Smash Bros. comes out on the Wii U and the GameCube controller adapter comes with it, you will only be able to use said adapter on Smash. That’s the official word from Nintendo this morning, anyway.
“The GameCube Controller Adapter for Wii U is compatible with Wii U and Super Smash Bros. for Wii U specifically – it is not compatible with any other Wii U software. The adapter is also not compatible with Wii and we apologise for any confusion.”
This follows-up a prior listing that stated that GameCube controllers would work for all games supported by the Wii Classic Controller, the Pro, and the Wii U Pro Controller. That posting seems to have been made in error.