Shader/Possibility to render nothing but the overlay

Hi there,

I'm asking this question because of the lack of correct uvs on a dynamic mesh. I will recieve an Input from a Webcam. This is the "Texture" on the background. Now I have a rectangle which vertices I can drag&drop an this background plane to rearrange the mesh. I also can add new points on this mesh. Therefore the uvs would be wrong if I display a texture on this "cutout" plane.

So what I'm asking for is a shader for a plane I will place all over my background, but which will not be rendered by the cam - except for the area which will be overlapped by the movable mesh.

Some kind like this see-through-hole-via-shaders-on-a-2d-plane but the other way around. Everywhere should be displayed grass, and only through the hole will the house be visible.

Maybe it could be one plane in front of the camera, which disable rendering of everything behind it. But my movable mesh should remove this effect. Currently I'm using this shader DepthMask, but now I have to do it the other way around...

I hope you can help me. Best wishes, Thomas