After some coding, debugging, and combining of code with some of my own glue (see my github), I threw it in an Arduino box with a bit of poor-mans strain relief.

Works with a Hori FightStick and Nunchuck, though should work with the classic controller and maybe other stuff, unfortunately I only had these two items to hand.

Video here:

The project isn't very complicated, and could easily be used to connect Wii Accessories to any other system with a little effort. PlayStation 2 for instance (SPI): or a NES (Someone's done it: