Hi RetroLefty 
You wrote "I am familiar with the 4-20ma DC 'sensor standard' industrial current loops.", and you are absolutely right. This is the standard I am referring to.
After some research I've come to the conclusion that it must be a touch of brilliance!
I believe that this can be achieved in a pretty simple circuit.
It usually does. Looking back when I have the answer ...
I tried to use a opamp to boost the input signal by generating AC and use a transformer and a rectifier to have a complete circuit "outside" of the Arduino system. The noise from the AC generator rendered the inputs useless. So that didn't work ...