Ok, I've decided on my next project.
At work, I have a magic trackpad for my laptop, and it's sitting right next to a USB keyboard.
A keyboard that has a USB socket on the side.
2 inches away from the trackpad.
Which eats 2 AA batteries a month.
A 5 volt to 3 volt LDO regulator will certainly work. The question is, how best to add it to the trackpad?
Well, OSHPark can make any size circuit board you want, of course. So how about one the exact dimensions of a pair of AA batteries? On one end, there could be a little "nub" with something conductive, like tin foil or something, connected to the positive output terminal. The other end would have a spring to push against the screw-in plug. For the negative terminal, you simply need some sort of springy metal contact that reaches either up or sideways or whatever to touch the side of the chamber.
The last challenge is how to get the USB cable in.
I've been playing with 3D printing lately. I'm thinking that the metal twist-in plug could be completely replaced with a printed plastic plug designed with a hole in the middle. The pattern for the plug would need to have the same thread around the edge as the metal one - perhaps the metal plug could be simply scanned in as a starting point.
Of course, this would only be for power - the trackpad would still use Bluetooth to communicate. But that's fine. I just want to save the hassle of changing the batteries so frequently.