The reason why 3rd-party cables are rare/nonexistent is there's a custom DAC in there. DAC stands for Digital-Analog Converter. The digital out port on the original GCs is for the Japanese D-Terminal system, and Nintendo had a special chip made that could convert that signal into YCrCb (Component). The chip is made exclusively for Nintendo by Macronix, they will not sell to anyone else. The GP is correct, the cube doesn't really support component at all, this is an external hack which keeps the cost of the cube down. Europeans got lucky, their cubes don't have S-Video but an RGB cable is like $5 there as it's just wires, no chips.
There is a good reason to buy two of these, also, other than eBay: if you are a soldering god or know one, you can convert them into a VGA Cable! Not a some ugly Redant junk that uses the composite video, true, pure VGA. It's expensive, though!
I am not at all sure these will work with the Wii - Japan loves new tech and I suspect that HDMI will displace D-Terminal in Japan, but I am totally speculating there.