Wow, this was great! I made a nice little night of it all last night, and it was definitely great, educational entertainment! I kept waondering the whole time how much the process of animating the legs differs when you're using Biped. I'm guessing it would be pretty similar, right, except instead of animating the foot link from the main object to the world, you'd just hit the FK/IK switch, right? Interesting to see that you don't use the render passes panel, too. This is the same way I set up passes usually, just save a different file and change all the textures to whatever give you the pass you want, but I always thought it would be easier using the render pass manager thingy, I just haven't tried it.
Very inspirational, too. When I have a free day, I may pull out my old transformer challenge project and try and finish it, since my transformer isn't much different from that roboball!