Hi, TEGs output voltage varies with Tdiff (Difference in temp between hot and cold).
Here is a graph some people have done. Assuming a 20oC Tdiff (body to ambient ) you will be getting about 0.5 of a volt.
Also assuming a 5% efficiency and you wanted to generate say 50 milli amps at 7 volts to trickle charge a pack of batteries (at 70% efficiency of charge) you would need to be drawing this many watts of heat from about 3 square inches of arm.
350mW *20 (5% efficiency of generator) * 1.4 (efficiency of charge storage) = 9.8 Watts call it 10 watts.
The human body at rest generates about 100 watts of energy. So 10% of you at rest body energy would be going through a small patch of skin.
I would guess that unless you are cooling the outside with say "Dry Ice" and are happy with a serious case of frost bite that perhaps a solar panel and/or mechanical generator might be best.
Sorry to disappoint, I love the idea, maybe the next generation Pi will use 1/10 the power of the current one and then it might be more feasible.