With the latest Plasma 6.3.4 I could finally enable HDR on my laptop which is Asus Zenbook 14 UX3405CA with intel iGPU Arc 140T.
For details please check out this commit :
Add the following line to the environment the HDR selection box will be appear in the display setting:
KWIN_FORCE_ASSUME_HDR_SUPPORT=1
tested with some random Sony HDR demo clip with MPV it works!!!
Cheers!