A helpful programmer has just released a first, very rough version of VidiiU Streamer, an application that allows you to stream MP4s from your Mac or Windows computer to your Wii U. Just download the software, extract it to wherever you like then run it. Choose a folder that contains your videos and when you start the server you will be shown a url you can plug into your browser. (The first time I did this the VidiiU window disappeared before I could read the url, so I had to kill the program then restart it, at which point I could see the url before restarting the server.)
VidiiU doesn't work for me, either because of the firewall in my router or because my Windows XP installation is a bit screwed up right now. But it's apparently working for others, so give it a shot.