The easiest way I can think of would be to have an additional camera that would display a simple plane. The camera would not move and the plane would be visible only by this camera. This plane would show the video, something like:
https://playground.babylonjs.com/#XCPP9Y#5393
You would probably need a bit of calculation to always have the plane at the same location in screen whatever the screen size (using an orthographic camera may ease the calculation), but I think it’s the fastest way of doing it given there’s no video GUI element.