Option for render region: Render region in last full render

I often use render region for a complicated part of a region i render again and again till it fits right. But i miss a complete render view of it. It would be nice if there would be an option "Render region in last full render". So if i use this option the first time it renders the full image if missing, or just render the region and combine it with the last full render too get an full and only partly updated render.

This would be nice for tweaking and can also be used to fix little mistakes in a final render without a complete render that takes more time.