This is an update for the beta branch of Desktop+.
New:
HDR mirroring works for both Desktop Duplication (default for desktop overlays) and Graphics Capture (desktop & window overlays) methods.
The full bit-depth texture is passed to SteamVR, but to my knowledge there's currently no HMD that supports HDR output with SteamVR.
HDR mirroring in Desktop+ is primarily aimed at working around the capture APIs not returning proper SDR contents while HDR is active in the OS.
This isn't enabled by default. It can be toggled in the Performance settings if " Show Advanced Settings" is enabled.
HDR support is considered experimental as it has only been tested to the point that it results in the same pixel data for SDR displays, plus a single user report with actual HDR hardware.
Please leave feedback if this isn't working on your system.
Since it's been a long time since the last beta build, here's a reminder how to opt into the beta branch: