Hi,
yes, i haven’t yet created a PR because i still have to implement infrared cleaning, and fix lots of stuff. I’m afraid i won’t make it in time for the 5.8 release.
In the meantime, i’ve created a dedicated branch on my git repo. If you have already built rawtherapee from source with the usual build-rawtherapee script, you can easily build this modified version by doing:
cd ~/programs/code-rawtherapee/
git checkout dev
git pull
git checkout -b rom9-filmneg_nonraw dev
git pull https://github.com/rom9/RawTherapee.git filmneg_nonraw
<it will ask to merge, commit the merge>
./tools/build-rawtherapee -b
(as described in this previous post )
Keep in mind that this is a preliminary version, so for example thumbnails don’t work, as the film base color picker. Also, it may crash a lot so please be patient 
You’ll notice i’ve moved the film negative tool from the “Raw” to the “Color” panel, just under Film Simulation.
To convert a negative, you can use the “Pick neutral spots” button and pick 2 different levels of gray (darker and lighter) from the negative, to auto-calculate the exponents.
You can use the “Reference exponent” slider to boost the contrast when needed.
After that, use normal WB controls to adjust the colors.
It’s still not a finished work, but you should be able to do some tests and get an idea of the results.
Please report back if you do 