How to create an overlay for use in another program?

I have been creating basic Oculus/Unity/C# apps for a few years now (personal hobby, not professional), using both the Oculus Plugin and the new XR Plugin system in Unity. I have recently gotten into MSFS2020/VR, but am having trouble with Vatsim/Kneeboard integration while wearing the headset. I would love to explore making my own "overlay" (interactive) that would appear in the sim in VR (or even other apps that arent mine). I have a good start understanding the MSFS SDK and API, but dont know where to start on injecting an overlay in VR. What API's could I look into that would let me do that? I'm guessing I would have to look into the OpenXR api? If Unity wont do OpenXR, I can do C++/Dlls as well. Any help pointing me in the right direction would be appreciated.