There is no such thing as “the Microsoft OpenXR loader”. I have no idea what gives you the impression that there is. I’m not even sure where you would be getting a “Microsoft OpenXR loader”? Do you mean the VS Nuget package for OpenXR? Because that ships the Khronos loader, bit to bit.
I use ActiveRuntime on my system to toggle between WMR, Varjo, Oculus, SteamVR, Pimax without any problems.