Quite impressive Jed ! Another approach that also gives good results I think ! 
I used these settings :
- Source gamut is ACEScg.
- Destination gamut is the one that comes by default in the Nuke script.
- Output primaries are P3. White Point is D65.
- EOTF is for sRGB display (so I suspect Gamma2.2 ?).
ACES
Jed’s per-channel
ACES
Jed’s per-channel
ACES
Jed’s per-channel
ACES
Jed’s per-channel
ACES
Jed’s per-channel
ACES
Jed’s per-channel
ACES
Jed’s per-channel
ACES
Jed’s per-channel
ACES
Jed’s per-channel
ACES
Jed’s per-channel
ACES
Jed’s per-channel
ACES
Jed’s per-channel
A few observations on my super limited testing :
- This looks like a super simple approach.
- It kinda fixes all the issues I have been pointing at (blue sphere does not get magenta anymore and light sabers don’t clip).
- Hues are better preserved I think as well (you can tell on the blue to magenta/purple sweep).
- And it kinda respects the ACES look in a way.
Now I’m going to play with the little tool you made Jed to see how the change of primaries affect the rendering transform. I need to do way more testing !
Congrats !