I've only personally tested this converter GC Controller Converter and it works perfectly. Darkprinny was the one that told me that these converters would work.
Since the Wii has the same controller ports as the GameCube and is just an upgraded GC then my guess is that if it worked on the original GameCube then it will work on the Wii (aside from 3rd party memory cards as they seem to get stuck - many threads on here with that issue).
If it was me I'd give it a shot - doesn't look too expensive to just buy one and give it a try. I took a gamble with my Converter as well (u never really know until you test it yourself) and it cost me about $25AUS.
However if it doesn't work, I am sorry but it is just a guess.