I can't really see the use of adding dof from corona to the backplate. That should be handled when photographing the backplate. Anyway the option should be there to turn it off just like we can turn off exposure for the backplates.
Yeah, but there's also case, where backplate is perfectly in focus, yet you want to do DoF shot. In that case, you want backplate to be blurred. This is quite tricky.
Are we talking about viewport/render background or actual mapped plane geometry?
For viewport background, there should be a switch, I agree on that. In case you don't need any dof and want to preview correct dof you actually need a background without any dof added in the render.
If you need dof on the backplate and/or want to exaggerate it you could use a plane and figure what dof fits best by moving towards to or away from the camera.
Also, there should be also a way to exclude ShadowCatcherMtl from dof calculations, only direct visibility, reflections and bump should still render with dof. Not sure if possible, sound tricky, but definitely the way to do it if you want correct resulst.
