Two years later and a new Truck with built in brake controller, but still this problem. Apparently the built-in controller only displays Volts, so I doubt it will detect whether one of the brakes is missing or not.
I was thinking of installing a small ammeter in the cab, but couldn't find one that would fit in any existing openings and wasn't about to do any major surgery on a new truck.
But with my new clamp meter a compromise is available. Whenever I check the taillights (usually before every trip), I will also use the clamp meter to measure the current flow in the blue brake line. On the Elkmont that brake line is easily accessible (maybe Too accessible) from the side of the trailer. And when I pull the emergency brake plug, there's 7.3A on the line. About what you expect for both brakes working. Working both brakes with the pedal should be similar. And so, if I should ever read 3.6A ...
This won't give me instantaneous readings while driving, but at least I won't be traveling half way across the continent with only one brake again.