These properties are available in CustomMaterial shader:
Using these properties it is possible to render objects in different ways for different cameras, but if the properties are equal for two cameras, then the two cameras can't be distinguished.
I suggest that camera index property should be added to cameras and CAMERA_INDEX to the shaders. Also it would be useful to have a sort of camera filter list in each visible node (Mesh, Light) to make the nodes visible only for particular cameras. Something like:
That could be used for rendering different stencils with using two or more View 3D objects. Then the stencils in combination with ShaderEffect could be used for glow/outlining effects, for real time cubemap reflection generator and other postprocessing effects