thanks so much for your insights.
ideally we're looking for cross platform compatibility. i guess, the differences in the platforms is the reason why its not implemented. However, since one of the last updates UNITY managed to support this feature native.
And yes, we basically need to play a video as a texture. Just fullscreen is not enough.
We currently using a open source engine, that does exactly what you describe with media foundation. but it limits us to directX and that's a big issue in future development.
We're aiming more towards WebGL which is not yet supported with XENKO anyhow, but basic WebGL already supports Video as a Texture out of the box. And given that WebGL (more or less) plays back on mobile devices, the platform gap might not be so big. But locking ourself in directX is a bad thing