Any way to dynamically Clip part of one widget when it's behind another widget (not in a parent-child situation)?

Effectively what I’m trying to do is have a “background” image that fills in the space between other widgets, and one of those widgets needs to effectively cut-out (clip) the part of the background image that the widget is in front of in order to allow free view of the game’s viewport. I want to be able to dynamically do this so that I do not have to manually cut out portions of a background to fit the space. I also do not really want to display the viewport as a render target and put that on the upper widget to make it visible, because that provides a bit of lag time and lower quality.

They say pictures speak louder than words, so if my explanation isn’t very clear, here’s an image that kind of describes what I want to do:

Another way to describe it is I’m looking to Clip “within” the bounds instead of “outside” the bounds.