Been done, not exacty what you are talking about, but the same concept. You can order stick on sights for you scope that are circles of different sizes, cross hairs, X's, what ever you want really. They also make scopes with a hole in the middle of them so the outside of the target is magnified (or reduced in some cases) and a small portion of the target is not.

Some people aim very well this way because they can focus on the target, not the pin. Like you said, it works very well for spots because you center them on the circles. Circle inside a circle, inside a circle. It works very well.

They also make pins sights that are small apatures.

Paul