Making an object A reflect hidden object B

I have some objects in a scene that I will need to render separately from the other objects. So I have them in a different layer. However, I would like these objects to reflect the other (hidden) items in the scene when they are rendered. Is this possible?

env map? maybe?

Add env map as reflexion for your object.
Render your scene with all channel on.
Save the env map in a file, turn channel of and give you file as color channel, you may have to tweak a bit to get a reflective looking but the idea is there.

If you are using compositing you might be able to use a rectangle covered with an Alpha=0.0 “blue screen” material to remove an otherwise-visible object from the camera’s view. This would be possible if the hidden object does not obstruct the camera’s view of the visible object.

You would shoot this layer to contain only the visible object so that obscuring the background would be a non-issue: the background would go behind everything regardless.

But it would all depend upon where the hidden object, the visible object and the camera are positioned relative to one another in space. If you’re committed to doing this shot, it will definitely be tricky.