So I made a test system, since there is so little data out there on how well this will work.
So far, so good. With a very few tweaks of MadVR from within JRiver Media Center 24, 4K HDR playback has been close to flawless. I get maybe up to 2 dropped frames as a 4K HDR video from my NAS is playing, but at the very start where it is unnoticeable. I have only tested with 23.976 fps material so far though, but that is really all I need to be flawless for now.
I find the Radeon software suite to be more or less useless, unless I am doing something wrong(?). There seems to be no way to change the refresh rate from within that panel. I was finally able to change refresh rates via some convoluted method before relegating the whole AMD experience as useless for video. I am quite happy with the results so far, but need to test with upscaled 1080p, 720p, and SD material too, as well as different frame rate files.
AMD Ryzen 5 2400G
Gigabyte AB350N Gaming Wifi
16 GB PC3200 G.Skill RAM
480 GB Adata SSD, which I do not trust
Mini-Box PicoPSU 192W
JRiver Media Center 24: Red October HQ; Hardware acceleration On, Refresh Rate Switching On
MadVR render settings: Delay Playback start until Render queue is full