Separating shadows and reflections via render elements on a matte object

[Joezeta]

I have a simple object laying on a matte plane.
I've been asked to separate shadows and reflections on a matte object to use them separately in post production.
Currently my shadows and reflections are baked in the rgb pass, so I cannot tweak them separately in Photoshop.
Is it possible to tweak some material or some render element to achieve that effect? Mind that I need that separation in the matte plane and not in the main object.
Currently I've tried with some render passes as reflection and rawreflection but I'm clearly doing something wrong, since in the specific render elements the matte plane's reflections are shadows are not visible at all.

[hermit.crab]

Not at this point. You can extract it in post by using the alpha channel with and without the matte object material's reflections.