I found out this trick in Carrara when I accidentily place a white cube (for reflection) in the render area, and saw how fast the rendering went. The area in the open space still does have te reflections of the whole scene, so that can pose a problem if you have to render higly reflective objects, then the shield could show up in the render.

But for that we could paste parts of the first render back or do some spiffy stuff in Photoshop or Gimp!