Thanks for the quick turnaround. I’ve downloaded and installed.
The good news is: I used RT to read a NEF file (from a Nikon D800), ramped up the “exposure compensation” to the max, turned off “Highlight reconstruction”, turned down "Highlight compression, so virtually all pixels were out of gamut, and saved.
Gimp and ImageMagick can read it. It is 32-bit/channel floating point.
Now the bad news: the maximum value in the image is 1.0 (on a “in-gamut” scale of 0.0 to 1.0).
Do I need to flip another switch in RawTherapee so it will write OOG pixels to the file?