I did a test by creating a mpv.conf file in my folder C:\Users\USERNAME\AppData\Local\PlexMediaPlayer
(I didn't had one, so I created one)
In this file I put the following 5 lines only, based on what I found here and there in various forums:
It's working. As expected, it didn't activate automatically the HDR mode of the TV, but it makes the HDR video more bright and contrasted. If I also activate the "HDR effect" (= false HDR) of the TV (special mode on recent LG OLDE TV), I obtain a result that is not so different than the same video played from the USB drive and who activate the "real" HDR mode of the TV. I say that by only comparing with my eyes, so it's not very precise :)
What is great is that these settings don't seem to affect non-HDR content.
I put 540 as brightness as it seems to be an average I've found in website that tested my TV LG OLED C6V.
I need to do more test to see what can be improved, probably by playing with the brightness and the HDR tone mapping.
Of course ideally, if we could disabled the HDR handling of Plex to send it to the TV untouched and the TV handle the HDR it would be better, but it doesn't seem to be possible for now.