Masking an overlay?

I'm looking to create an overlay with a portion cut out that has the natural rendering of the scene. This would be easy enough if the position of the object being highlighted were in one place, but the camera is movable. So far I've found the solution seen [here][1], but it doesn't really give me the option of a mutable center. To better illustrate what I am trying to achieve, please look at the images below. Here the scene has a blue overlay with the rounded rectangle cut out to show the region of interest. Each object is a different size, so the "hole" needs to be scalable: ---------- I am working with the free version of Unity, so using something like GUI.DrawTexture is out of the question. Right now I'm just looking for a direction to head, so any suggestions would be appreciated. [1]: http://answers.unity3d.com/questions/14770/creating-a-circular-progressbar-timer.html