A simple board with regulated output and USB-B connector.

Washable ?

I will second this!

I third this, also not just having a small battery with a connector for Tinyduino/lily but also making a secondary bus where these could easily be chained together with minimal effort, so if you need new power you just plug more batteries into each other in sequence with a connector similar to the Tiny's bus connector, snap and go!

I really like this idea, especially if multiple battery shield can be stacked to add capacity.

It would be great to add a super-thin Lipo to the range of batteries.
This would allow some new form-factors / credit card sized electronics :-D

E.g.
https://www.powerstream.com/thin-lithium-ion.htm#1101
Personally I want: PGEB016144 ultra thin battery 1 x 44 x 61 mm 200 mAh (it's actually 180mAh) ...
https://www.powerstream.com/p/PGEB014461.pdf

These are expensive in low quantities, making it difficult for us to think of adding them to our standard line of lipos as a tiny business without having a specific reason or market opportunity.

Cheers,
Réna