I don't think the Mill is designed for this use case, exactly. Optimizing peak performance per watt is very different from optimizing for low power. The latter would be dominated by shutting off circuits, different sleep states, lowering leakage and voltage, etc.
Something like a GreenArrays chip might be a better fit for that direction (though very unconventional).
A good way to learn about low power is to play around with an MSP430 chip. TI was selling trial kits for something like $5 or so. (Cheaper than the included USB.)
Something like a GreenArrays chip might be a better fit for that direction (though very unconventional).