For the time being I created mask assets in Photoshop and use those to define the frame of the objects. It's hacky but it works.Wouldn't the source bounding box be then the "edge" for calculations? And if so, this should be relatively cheap in cost due to the simple geometry involved?
I ran into the lack of output trying to put a border "inner glow" around my video source. Disappointed but I can wait for a proper decision.