It's not that hard to use a screen capture and overlay, though the method used in Bryce insures everything "line up" without any effort.
I may be wrong and the Planetside staff will tell me, but it seems everything is already in place to simple overlay a crop render (or not.) Just don't clear the render window and render the crop area. This would make it really easy to compare changes to area of interest.
By the way, I'm not a Bryce fan though it may seem like it. I can't believe how bad it looks compared to TG2. Just that plop render was so convenient.
We've come a long ways!