News from mspoweruser is that Microsoft just released an app that enables MR Headsets to use OpenXR apps.
From the store for free:
OpenXR for Windows Mixed Reality provides functionality which is required in order to run OpenXR applications.
I’m not sure about the System Requirements listed there… Xbox VR or cpy/paste mistake?
Your device must meet all minimum requirements to open this product
OS Xbox One, Windows 10 version 17763.0 or higher
Architecture ARM64, x64