That would only work if you are in a 2D scene. In a 3D scene the reflection shows areas that are not there.

Take a can (coke or whatever) place it in an angled view. Take the buttom of the can. The reflection would show the underside (the concurve of the can) that the front camera can't see. It would look so unreal that you won't use it.

to understand the problem with reflections.

You might want to take unreal engine and make a composition there. This might be more target oriented.

Thanks for the replies!

I am actually tasked with making a live comedy stream with "deliberately bad" 80's greenscreen look, so perspective is no issue :)

I understand the virtual set thing - I can see that I can easily mirror the UVs to mirror and place the cloned input.
That just leaves the colour correction of the "reflection" / "Shadow" channel.

I would like to make (bad) fake shadows too - so I need a way to make the flipped UV dark
I can make the reflections and shadows soft / blurred by dithering the UVs

So I nearly have a plan - just need to darken the second instance