Well it's taken me awhile but I've gotten to a point where I can test this. I've powered the board from a 7805 5v output direct to the 5V pin on the arduino... From what I see even though this is usually taken as an output, it's parallel to 5V supply on the board so power could be provided on this pin as well. The arduino turns on and works as expected. I have observed 3.3V at the 3v3 pin while doing this. According to the docs:
"The FTDI FT232RL chip on the Nano is only powered if the board is being powered over USB. As a result, when running on external (non-USB) power, the 3.3V output (which is supplied by the FTDI chip) is not available and the RX and TX LEDs will flicker if digital pins 0 or 1 are high."
this should indicate the FTDI chip is powered and the USB connection to computer should be functional. I would just need to create a special USB cable that has the +5v wire disconnected. I'm not sure what would happen if two separate 5v power supplies were routed to the nano at the same time in parallel. I would assume that it would just provide more current but being two separate supplies I'm not sure.
I am confused however at one thing. I powered the nano by the VIN pin with +12V. I checked the 3v3 pin and I observe 3.3v available at this pin when powered this way also. From the docs I quoted above, this should not be. Perhaps this means the FTDI chip is powered in all circumstances and USB connectivity is available to a PC in either scenario of power delivery? I assume the GND from the USB cable would need to be left connected if using an external 3v source.
Any thoughts?