I did read your topic just now.
But to me it don't seem to be necessary to search for extreme small power consumption.
I am currently working (also in spare time) on a batterieloader for reloadbatteries, based on arduino.
The purpose is to disconnect from power supply (in my case 230 ac) if the load is complete via a optocoupler.
But in your case i would work with solar power (a small surface should do) and reload batterypack (4x 1.2 volt cells do very well for this purpose).
How it would best work in case of solar power supply : that needs some work out, because i also had some problems with to make it all go smooth.
But mainly i think a combination of the solar cell + a constant current source toward the batterypack should do.
In this way it must be possible to build easily a device that works even over a year or even two and weighs less than 1 kg.