Depending on exactly what effect you’re going for, this could be either quite simple or pretty tricky. If all you’re trying to do is have a video in the background with various other elements in the foreground, you can do that with just a VideoTexture and a Layer.
With something like that, you could have whatever meshes you want in front while the video plays in the background (a webcam feed is used in this example). You’d have to add your own mechanism for synchronizing what the foreground and background are doing, if that’s something you wanted; but the basic technique is quite simple.
If, however, you’re wanting to do things in the foreground based on what’s happening in the background video (and without pre-set scripting/processing), that gets into the realm of computer vision/AR, which is a bit trickier.
This is an experiment we’re using to explore some AR capabilities in Babylon. Try clicking somewhere in the video feed (ideally somewhere visually identifiable, not just a spot on a blank wall, for instance) to pin an annotation to that point in the video. This experiment helps to show what’s possible (and what’s required to make it so), but keep in mind that this is a very early experiment and not something to take a real dependency on yet. However, if that’s the sort of capability you’d be interested in, definitely let me know!