Impressive work being done here.
Some thoughts on solar powered applications. I think it's safe to say current draw in idle mode is 100..120 mA, and it's not just the processor that eats it but also other components. So I guess it'll be hard to drive that number down with firmware changes. And I think hardware changes with SMD's are a no-go, because you'd need specialised, expensive, eco-unfriendly equipment. So, please correct me if I'm wrong, 100..120 mA in idle mode is as good as it gets.
In solar powered application you normally do tasks x, y, z and then go hibernate for some amount of time, wake up, do x, y, z, perhaps compute the next hibernation interval, hibernate, and so on.
In hibernation, you want power consumption to be close to zero. 120 mA (~0.5 W) could still be way too much in many situations.
So you'd need an 'independent' external device that reads and stores that hibernation interval, shuts power to the Pi off, turns it on when the time comes, waits for the next interval to read, and so on.
You could do it with a low power relay and perhaps an ATtiny13A, which draws only 24 uA (microAmps
) in idle mode, 190 uA in active mode.