uController dual-supply (low stand-by)
Have a look at the LM5164 100V in, 5uA at 40V Iq?
Depending on the controller, you can either linear from 12V to 5V (low power) or use a small switcher (higher power).
Note: most buck converters efficiency drops off when you start talking sub milliamp currents and 12V micro-watt BUCK is still not that common.
Simon