I think Unity by default keeps the transparency in its render buffers until everything is finally out on the screen. If you set up two identical cameras in the same spot, let one render normally, and use a custom fragment shader on the other which, regardless of all inputs, outputs e.g. white, then you can map those cameras to two different parts of your game window and get a side-by-side actual-render and mask-render (provided you have an empty background and the color you use for the mask shader is different from the background color).
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