For a moment this sounded great, and I was about to cast a vote for it, but I see a serious technical problem with this request.
This idea may work fine for a FLAT "mirror" object, or the "camera viewfinder" that you mentioned, but not for other shapes.
Consider a cube. Each surface on the cube faces a different direction. If you look at the cube from an angle where you can see 3 faces, you would need 3 cameras because each surface faces a different direction. And each camera would need to be mapped to a specific face on the cube, which is only a portion of the cube, so it would be wrong to map the camera's view onto the material for the entire cube.
You might get some interestingly creative and artistic results if you mapped the view of a camera to the surface of a cube or a cylinder, but it would not result in proper reflections.
But for something like a stationary mirror (bathroom mirror, for example) when you have a stationary primary camera, it could be useful.
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