I recall reading about a project at a university (MIT maybe) where they wanted to capture power as people walk down a sidewalk.
It would be interesting to do an energy study - comparing the efficiency of the human engine with other methods of generation. One thing to watch out for - the energy value of a calorie is different depending on how you use it. People who do more work need to eat more -more food requires more energy to produce. Is that better than just generating the energy directly? This is not to say your idea is bad - not at all. It's just something to consider.