Yep you use the plug from the Xbox composite cable and the component wires from the 360 component cable set. People sell them on ebay made up like that, it's a common mod. Both cables are fairly easy to source and you end up with a high quality cable.
The Gamecube component cable can fetch a handsome sum of money now, at least over here anyway. I've seen them sell for over $150aud.
Never had one myself but I do have GC Plug (GCVideo) which is HDMI using the digital port, compatible with both PAL & NTSC consoles. I use a generic remote for the menu (actually sourced from old SD STB).
