Just from top of my head... I don't think this works at all as you would like, since decal is just a texture used by material/shader.
A bypass//
So to create yourself a solo decal pass for masking in post, disable all scene lighting and just apply light material to decal (or any matter you want to have mask made for) then render it out.
Could also make a Take, if you're not sure about the camera position and mess as little as possible in the future.