I am not that stubborn but work has picked up; I may get to posting on the weekend.
The process is as follows:
By the Calculation of the same gray source as normal one simply produces a Channel representing the luminance (of the image or the Layer).
The Histogram indicates which is the first color value that is non zero, so actually represented by pixels.
That value used in Threshold on the Channel makes those pixels white and all other pixels black.
Then one can manually load that Channel as a Selection (manually by cmd-clicking it).
An unforeseen issue: In Layers that have transparency the wholly transparent regions are represented as white – but this is solvable.
Other than that transparency is disregarded in the Calculation – so one should bear in mind that a light pixel in a very much transparent region still counts.