Could it be useful to use these to charge long term batteries to be used millennia from now? So that future generations living centuries from now can have easy power when the fossil fuels run out?
No. Batteries only last a few years before the chemistry degrades.
And anyway, microwatts are useful in certain restricted circumstances, but are easy to come by. A solar cell illuminated by starlight gives you more. Needles in a lemon gives you way more more.
