Is there a reason why a isolating resistor won't work?
After thinking about it I would probably want closer to 10ohms to prevent current limiting from the port to the board (~500mA).
If there is a difference between the 2 sources of voltage the resistor will drop the difference, the load will only see the lower of the 2, and the drop on the resistor through the load/board is negligible, far better then any diode would give you. The feedback current on the resistor is also going to be very small % in relation to the regulator capabilities.
I was never that good/interested in thevenin theorem, but this seems like a basic application of it, so if I'm wrong, how so?
P.S 40 years doing what with electronic's? If its 40 years of hobby I could see you doing it wrong. At this point 40 years of actual design in a job seems very unlikely.