After a crap ton of thought (this topic is intriguing me) and playing around with a few different photos, I think the in camera HDR can be somewhat duplicated using the Lightness value (HS
L) of the RGB channels - for example:
HDR Normal:
-3 | 0 | +3
Correctly Exposed Snap:
L | R=0, G=0, B=0 (where 0 is neutral/normal value)
HDR Strong:
-3 | 0 | +3
Correctly Exposed Snap:
L | R=+20, G=+20, B=+20 (where +20 is the adjusted channel lightness value)
When HDR merging the above, it seems to mimic what the "in camera process" does - maybe not 100% but it is close. I am sure there is more to it like contrast adjustments and such; this would be a good project for someone to tackle, may even lead to stumbling upon a new HDR routine...