Thanks so much, Nick. Finally got around to trying the LUT, but having a strange issue. Pretty sure I placed it in the correct location:
but when I apply it to the timeline thumbnail I get:
....and it has no effect on the clip.
Running version 12.5.4 on Mac OS. Using ACEScct v.1.0.2., REC 709 ODT.
Also tried applying on the color page, and in Project Settings, to no effect.
That said, I was ultimately able to fix the artefacting by pumping in 80% Green tint, meaning that, in the final analysis, it wasn't actually an ACES problem. So that's a relief. And possibly the reason I was seeing no effect on the image or waveforms.
Not sure, though, because of the error message.
Peter and Thomas, thank you for testing.