I've been using Endeavour with KDE and HDR on for almost a year. I had to install the "vk-hdr-layer-kwin6-git" package and now HDR works on desktop and in mpv/smplayer/haruna for both hdr10 and DV content. Games also work, but they require some steam launch arguments which aren't exactly user friendly.
Ideally this will "just work" in The Future™...
Absolutely! I recommend reading through wiki.archlinux.org/title/KDE#HDR and at the moment I am playing through The Witcher 3 with these launch options I got from someone in a review on protondb.com: "mangohud VKD3D_DISABLE_EXTENSIONS=VK_KHR_present_wait gamescope -f --force-grab-cursor --hdr-enabled -W 3840 -H 2160 -- %command% --launcher-skip". Not everything here is necessary, but AFAIK to get HDR support in Steam games you have to launch the game with gamescope. So be sure you have that installed as well. I played about half of Ghost of Tsushima without HDR before I got it working and noticed the difference right away, but Witcher 3 is a good test because of the in-game HDR settings menu. I'm guessing that menu is unavailable if you don't have HDR correctly configured, so it's easy to see if it works.
As I mentioned, not exactly user friendly! I also have a glitch that happens if my HDR monitor is turned on after boot, or has turned off bc of power saving and I turn it back on. I get artifacting when a program goes fullscreen. My fix is turning off and on HDR on that monitor. Good luck!