Importance-Unimportance map for rendering

Hi all. So you probably know, that every HQ render with deep shadows (or caustics) is long. And you can see that render takes some pieces again and again, buthout any point, because it is already looks good.

So we have render layers, branched pathtracing, where we can add importance by object-groups or ray-types. Also you can see on the screenshot solution for rendering custom-shaped part of view (shader on the blackboard mixes holdout and transparent). So I can render only noizy shadows on both objects and compose with main picture.

But for static images or scenes with static main light it would be great to have Unimportance map too. Just to say to renderer «Ok! Thanks! It is already looks great and no need to calculate it again and again, Take other parts!».

Such map can be done over prerender. 2D-map is good, but 3d-map is great: ability to paint problematic parts over scene. i.e. add-on «Texture Atlas» can make atlas texture for all scene. And there is no problem to join proxy objects into one mesh and unwrap fast (with smart) for painting such map

Why manual map? Now I am learning with YafaRay. It have awesome algoritms and adaptive sampling too. But! It just searching sharp pixels. No matter, if they SHOULD be sharp, renderer trying to fix them. Only human can know is an object looks ok, is it should be noisy or smooth.