FYI Dave, this is how RIMM is used in ACR according to ICC:
http://www.color.org/scene-referred.xalter
Quite an old publication, some might say.
xalter-speak is not real easy to understand either - a price pedants have to pay, it seems ...
P.S. I downloaded ICC's ISO RIMM profile (ISO22028-3_RIMM-RGB-exCR.icc) then opened an image in RawTherapee with it as the 'input profile'. I kept the working space as 'ProPhoto', made no adjustments and saved with an output profile of 'sRGB2014'.
The color in the saved image (mostly foliage) was slightly more pleasant to my eye. In RT, one can save the entire processing sidecar .pp3 file and re-use it on a new image ... tempting ...
I also tried their linear RIMM profile (linear_RIMM-RGB_v4.icc) but, used as an input profile, it seemed to render with a pretty high black level. Bloody v4 strikes again ... ??
P.P.S. There's an ISO flowchart
here which clarifies the difference between and usages of scene-referred and output-referred encodings.