The port is only present on "Family Edition" wiis. These wiis have no gamecube ports, and a board code of RVK-CPU-01 or RVK-CPU-02. It's traces run back to the gamecube memory card data lines, and gamecube player 1 and 4 data lines. It has no data traces that the Wii didn't have. Check link in my signature for more info.
The port seems to be for debugging. "Save mii" needs access to those lines to work, and i imagine Nintendo does to as part of their manufacturing, programing, debugging, or repair process.