Several times in UE4 documentation, it refers to LUTs as only working in display-referred space (such as Rec.709 sRGB) and are thus unsuitable for HDR workflows.
However. I can take a pre-tonemap (scene-referred) ACEScg EXR out of UE4 into resolve, set resolve to ACEScg configuration, do color grading, and generate a LUT from resolve.
Could you not, theoretically, create a LUT-based pipeline where a colorist gets scene-referred values, gives back a LUT based on scene-referred values, and then the engine takes the LUT, does the transformation, and then tonemaps to whatever display it needs?
In short, I don’t see why using LUTs is inherently impossible for HDR. Does anybody have more information?