"Selective rendering"? Couldn't you get pretty much the same results if you set up or added a special camera for zooming in on the area you're interested in, and switching to that camera for test-renders? Or even easier, just storing a preferred camera position in a dot so you can switch back after zooming it in for a quick render? If you set render options to a relatively small render, it wouldn't take very long. ...Or perhaps I'm just not viaualizing the problem properly....