** revised 7/16/15 **
See http://dicks-arduino-nano.blogspot.com/, post 16.
As I am wont to do, I posted without enough testing. Hopefully, corrected now. I'm surprised (shocked?) at how little interest there is in this topic. Anyway, the setup pictured only works if the USB out draws less than 500mV. None of my Nano test setups take this little -- nor does my Particle Core. So regardless of 5V USB input the the system fails when the battery runs down (USB output drops to 4.1V).