Check the discussion on the Wii Homebrew Channel. Many media player options are opened up once you softmod your Wii, including what you propose to do. Many links to tutorials and the necessary apps are in that discussion. I use Geexbox and MPlayer on my wii, but I haven't hacked it to use an external hard drive. I use a USB flash drive or an SD card.
Well, the problem is that I haven't hacked my wii yet. What I'm kind of looking for is conformation that it's possible. Anyone here that has done this before?
What version is your wii.