Rechargeable Battery for TWINE2
It seems that with most products going to Li-ion or Li-po batteries, it would be wise to integrate it with TWINE too. After all, a perfectly good charge socket is already there (Micro USB), and there's still plenty of room on the PCB to include a charge chip. A Li-ion battery of a similar size to 2xAAA's would have oodles of power to run it for a considerable length of time.
Whether to have the battery permanent or changeable is the question though. Permanent is simpler, cheaper and could be more powerful for its size, but it has the difficulty that you will need to charge it where it's located which could be awkward for some people. However changeable allows a user to go and swap the battery at an instant when it's getting low.
I might have a look at testing the idea anyway, a simple charge circuit would be easy to incorporate into it from the USB socket, and as long as TWINE doesn't get upset at having 4.2v at its battery terminals, it should work just fine.
[Edit] Just found out recently that most of the chips are rated up to 3.6v, so there may be issues going up to 4.2 at full charge.