If only the Wii had at least 4 USB ports (maybe more for other things too, like the LAN adapter and whatever else they're planning) and if any USB controller could be used, then we'd have a huge slew of gamepad options. I use a convertor for N64 and PSX pads on my PC, and there are adapters for pretty much every major console controller, even the older ones.
(I'd like wireless versions of the original VC controllers that don't require the Wii Remote to function, actually, but maybe that's asking too much.)
Getting back on topic, I would imagine that if these adapters convert input into proper GameCube input, it should work. Input is input. Though I recall that someone in another topic was saying they couldn't use a particular controller for a TG16 game, so I guess the input signal isn't as simple as just sending the letter "A" to the system when you press what should be the A button or whatever. It really should be, but things are never as simple as I imagine.