No, full transparency does not work, unfortunately, as I am getting undesired effects with it.  Maybe it would work for extracting some individual passes.

I am getting a bit of the result that I want (not fully yet) with just the shadow catcher that is catching global illumination too. I am not using any lights at all and lighting the whole scene with just the shader glow and ambiance. No reflections, though :(.

But I am still getting 3D object that is way too sharp, saturated and clear compared to the image. Are there any compositing tricks for dealing with that?