How to show part of the screen only for specified sorting layer?

I'm using Unity 4.3 sprite system, and I need to render only part of the screen for specified sorting layer. I know how to do it partially - by using another camera with greater depth and it's normalized viewport rectangle. It works ok, but layer I set for this camera now looks like the topmost sorting layer (becase of depth). So, can you advise me how to show only part of the screen for certain layer without putting it on the top?