In my mind I've dreamed of building step-up converters that let me put one battery in devices that run on 3 or 4 cells. Incandescent flashlights, LED flashlights, a clock, a thermometer... I could deliver the full voltage the device needs, with only the one cell to test and/or replace. However there's the headache of fabricating an enclosure for such circuits. It ought to be a small battery-shaped shell whose electrical contacts match a battery and also the ends of the compartment.
You might manage okay with a few cells and a buck-boost converter. And with less wasted power. As you know the 317 regulator is resistive drop. And doesn't the 317 subtract a volt or two?