I've read people say that the digital port on the gamecube is a standard in Japan, I can find no evidence of this though. AFAIK the plug is nintendo specific and the HD connector in Japan is a D-Terminal. Also, the D-Terminal is an analog connection, so again AFAIK there is no direct way to connect anything to the digital port and display video.
The chip in the cable has the following printed on it:
To take the cable apart, you just need to bend the thumbs that are holding the top to the bottom, there are 6 holes in the bottom of the cable, if you insert a small screwdriver in there, you can feel the thumb and bend it back to pry the top and botom apart.
Once you have the top off, you can remove the metal shell and expose the board
With the cover off you can clearly see the red green and blue colored cables for the component video coming off the board.
I found one page posting a pinout for the digital connector, I don't know if this is accurate though. In case it goes away here it is: There are 22 pins
|DetectCable (pull high)
Your best bet for making something work is to find the MX chip and solder something up on the motherboard, the connectors are going to be just as hard to find as the component cables. I was experimenting with a LMH1251 since it does all the same work as the MX chip, but without knowing the particulars of the MX chip it didn't go far. Maybe if I find a gamecube for 10$ I'll try soldering something up.