It's really awsome to read! - you know a lot about bit and iamge quailty - i gotta find more information about that myself..
" sure, the increased bit depth provided by having raw data available "
- The RAW file is more than just information from the bit, It's the unprocced image from the camera! So it holds contrast, dynamic range, white balance. So it's more than just the extra bit - It's all theese extra information the picture had, before it got "backed" intp a jpg.
"So, to make a luminosity mask that will separate the two, one can use curves "
- Not with this method - You need to use the camera raw editor, with an actual raw image. Curves can't acces theese dynamic range data that a RAW object in a smart layer holds.
"the mask is minimal unless the tonality difference between hair and background is very small,"
- Am i lost? Don't you mean very high? With more difference it should be easyer to differenciate..
Haha this is really getting technical.. Good debate Imo 
Thank you for the critic !
