Interesting findings Chris. I did notice that too with my 2013 CX-9 - if you stopped and started quickly, it did remember the last setting.
So what would entail making the USB "hot" all the time? Just moving to a different fuse or something? Would there be any battery drainage concerns with that?
It would be pretty minimal drain, especially if it were a drive plugged in (rather than a phone which would be charging). I have my doubts about it being this simple, but it would be nice. I could look though the service manuals for the 13 to see if it has the wiring diagram listed, but it would be far easier to just find out from someone whom has a 13 if their device still charges, or if the LED on the usb drive is still on when the car is powered off.