A few years ago, Kelly Wu designed a USB host enabled microcontroller board. (It was based on a PIC24FJ64GB002 plus some regulator circuits, so it's really more of a breakout board.) It never got popular enough to make it to mass production (originally intended for some USB datalogger application), but it did have a very interesting feature: the 5V USB supply could be margined from about 4.5V to 5.3V by turning a trimpot. The theory was that when using a USB Flash drive that ran all its internals from a LDO (nearly all of them nowadays), reducing the voltage would theoretically save a little power, or the voltage could be boosted to compensate for long cables.
I have managed to build a DIY clone of the Kelly Wu and got it connected to various Android devices, which got me thinking: what's the rough percentage of devices that use switching chargers as opposed to linear chargers nowadays? In that case, a somewhat lower voltage would work great for keeping linear chargers cooler, but would slow down charging on a switching charger. Am I correct that almost every Android device made nowadays uses a switching charger?