I’ve been having some luck with OVR Toolkit. It’s a SteamVR overlay to display and control windows or desktops in VR games.
You can add the game’s desktop and adjust it so overlaps the vorpx screen in immersive/cinema modes. Then set it at 5FPS/eco, 0% opacity (with a small hack) and disable touch mode in general settings.
This is the result I got with The Witcher: https://www.youtube.com/watch?v=rS_eaBgu9PE
A few words of caution:
Games meant to be used with a mouse can have small UI elements. If you ever used controllers to point in VirtualDesktop or WMR/SteamVR desktop viewers you know they are not ideal.
OVR Toolkit is a paid app, I use it for other things and consider this use case a bonus. The experience might not be as expected and future updates might make it unusable since it’s not an intended purpose.