transparent object, so everything behind it appears in grayscale

i would like a sort of transparent glass window that not just adds a color. It should have the effect that everything behind it appears in grayscale. But not by actually changing the shaders of the objects behind it, because there is another camera. Also the object should be round, so an cameraeffect should be not possible, right?.

Additionally it would be really good to have a way to mark certain objects to be an exception to that grayscaling. So they still appear in color if looked at through the glass.

Is something like this possible?

It should be… I think what you need to do is render the glass objects to a mask texture, then use that mask with a postprocessing shader to convert only those parts of the final render to grayscale.