Joule Thief - Why only single celled?
A joule thief is nothing but a primitive switching regulator. There is nothing about it's design that restricts it to a single 1.5V cell. Most use a 1.5V cell for size reasons. And because a single 1.5V cell, at 1V, can easily be boosted to 3+ volts to power a white or other 3+ Vf leds.

I've used a joule thief self osscilating circuit with 2 AA to power an LED for months without issues, with a random ferrite ring and Telco wire. Nothing optimized.

If the cell is full, the oscillating circuit gets saturated faster and less energy is needed to boost it to saturation. It will simply last longer.