Sometimes when the render line is in a very intensive location in the scene (example: while calculating a glass object within a volume surrounding) Bryce is slow to respond to the halt. 

I have discovered that to do a plop render, you don't even have to wait for the first pass at rendering to complete.  Just click the render button after the render has started and "plop" out the section you need.

After re-reading your post, are you saying you are moving things without halting the render?  I didn't think you could do that.  I always halt the render (click render button or click keyboard esc twice)