Temperature differential is the hardest aspect of this, even if they somehow manage to pipe the heat around the edges any time the user is wearing long sleeves

Temperature differential is the hardest aspect of this, even if they somehow manage to pipe the heat around the edges any time the user is wearing long sleeves you're going to wreck the heat dissipation - if they could use the top surface of the strap as a heat sink then at least they could get the surface area. Using a Marlow TEG harvester is out of the question as they are too big (and work best at much higher temperatures) but I guess one of the micro scale TEGs made by Micropelt could maybe possibly work - last I checked they were still in beta and when I referenced them in my lit review 7 years ago they were just getting going so it's been a long slog. Even then, as others have said, the harvestable energy from the surface of the skin is so small that it would have to be pretty limited in capability. The best bet would probably be an arsenal of energy harvesters, micro-scale vibration energy harvesters (mems piezo maybe), a magnet in a coil to get walking energy and then also TEG. Could land up being pretty huge. Either way, so long as this isn't a scam, good luck to them.