Just watching some behind-the-scenes work with greenscreens, and occasionally the filmmakers would apply an overlay to their field monitors to line up shots. For example, if they were shooting two people on a greenscreen soundstage, they could superimpose the background footage to help line the shot up correctly.
Does anyone know which recorder/monitors on the market can do something like this? (Say, play two video streams superimposed over each other? Or load in a graphics file to superimpose?