The keyword in this list are only "reserved", this meant they cannot be used with another signification.
But they are not mandatory and processing software must expect they are not present. In fact I never see any capture software that add this keyword because it make some sens only after calibration to some physical flux units.

Instead of adding a dummy value like "adu" it is better to report that to astro-pipelines so it can be fixed to work without this keyword.