6hearts:
Another idea: could I just stick two 5V power banks in each leg (I think they should fit or I can make them fit) and wire them to the 3xD battery holder? Then each leg would have 5V instead of 4.5V and it would be easy to charge via two USB's. Any downsides here?
A fresh Alkaline battery can be as high as 1.62 V so 3 x 1.62 = 4.86 which isn't quite 5V, so it's still a bit of a crap-shoot -- if you wish to play it safe, put a schottky diode in series, like a 1N5818.