[Soli]

Kodi did not handle bt.2020 (and even bt.601) correctly. I have fixed this for platforms using OpenGL (OSX, Linux X11, Wayland): https://github.com/xbmc/xbmc/pull/13428

I hope maintainers of other platfoms will fix this too.

Then at least the first step is there. Could you implement autoloading of a chosen lut file whenever Kodi encounters a hdr/bt.2020 file? 
I don't think it would be neccesary for a "convert HDR/bt.2020 to SDR/bt.709 option since this would be a function of the lut file. (But maybe it should be included in the future if Kodi for Linux and Linux itself supports HDR passthrough) Look at how madVR does it. You probably want to replicate that.