Is there a 5C difference anywhere in the body? Skin temperature to blood temperature, maybe?

That's what I was thinking. 37°C inside vs. up to 32°C (90°F) outside and you still have enough temp gradient to produce the power. All you need to do is install this heatsink on your chest :)

And to satisfy precedent, pipe any leftover power after running the pacemaker to a ring of white LEDs.

Testicles versus main body is the classical example. Sperm production and conservation just works better around 32°C.

On a more practical note, I wonder how feasible a thermoelectric powered earpiece would be...

How does that works? For a period of time before you try to get someone pregnant you should try to keep your testicles at that temperature to get the best quality possible?

Yes. Boxers. Loose pants. No hot baths or saunas. Good luck!

There are likely papers on the exact effect over to be found on google scholar.

Thanks. If that energy is stored in a battery or capacitor, how could it bleed off any excess charge (in your example 10 v 12uW). Is this a thing that’s is problem anywhere?

Getting rid of a few microwatts is never a problem. If these things behave like Peltier elements, they'll simply float their voltage up to a design maximum then stop extracting energy as the electric field balances the energy of the electrons they're trying to extract from.