Okay I can think of two routes here. I hope they makes sense on a technical level as I’m also still learning 
so anyone please correct me if it’s rubbish!
One makes use of a full ACES workflow and the other ditches the RRT+ODT in favor of ARRI LUTs. Both can use the ACES 1.2 OCIO config but the latter isn’t really an ACES workflow, I just used it for convenience of color space conversions. For the ARRI one I chose the ALF-2 LUTs (that’s what the others are right?) instead of K1S1 because the Classic is only available as Rec.709 from what I could see on their site.
The K1S1 ACES LMT is not something official but was created by @priikone more info here.
Pekka describes that it’s best to also use the RGC because the LMTs preserve high saturated colors.
The LUT based RGC I took from the StdX config that @Derek suggested in his post above.
This is a 1d shaper and cube LUT so can be used in our ACES 1.2 setup.
This ACES setup comes close to what K1S1 does except for very bright colored lights.
I do want to point out that regardless of what route you take, when it comes to lighting and rendering in Blender, it’s virtually pointless to do this with that ACES RRT+ODT I suggested earlier because you are modifying the look of ACES in Natron. Unless you create that same setup as a look in the filmic config you’ll get a different result anyway so maybe viewing with filmic is already sufficient. Maybe figure out for yourself what you find most sensible to do there.
ACES workflow
→ Blender render in .exr (sRGB/Linear)
→ Natron all media Read converts to working space (ACEScg)
→ Comp and grade linear operations
→ Convert: ACEScg to AWG/LogC
→ Comp and grade in log
→ Convert: AWG/LogC to ACES2065-1
→ LMT Reference Gamut Compress LUT based
- aces_gm_lut_shaper_to_linear_v01.spi1d dir: inverse
- aces_RGC_33.cube
- aces_gm_lut_shaper_to_linear_v01.spi1d dir: forward
→ LMT Emulate K1S1
- M-ARRI_Classic.clf
→ Convert: ACES2065-1 to ACEScg
→ ACES RRT+ODT to Rec.709 / P3D65
→ view /write
ARRI workflow
→ Blender render in .exr (sRGB/Linear)
→ Natron all media Read converts to working space (ARRI Wide Gamut / Linear)
→ Comp and grade linear operations
→ Convert to AWG/LogC
→ Comp and grade in log
→ ARRI LUTs to Rec.709 / P3D65
→ view / write
Here are the nodegraphs, hope it’s somewhat readable. The conversions after the read nodes are just for clarity, you could do this on the read node themselves ofcourse. If you’re interested I can also share the .ntp’s.
