Is there a way to fetch albedo/spec/roughness/AO and normals (RTs) in a deferred frag. shader?

The title pretty much covers it.

I want to create an opaque per-pixel fragment shader that:

• Will be rendered after all normal opaque surfaces but before all transparent surfaces

• Will have albedo, specular, smoothness, occlusion values and normals fetched from the deferred render targets (for each given pixel)

• Will output those fetched RT values

Basically, this will make the shader completely invisible, as it will just write back the content of deferred render targets. This is the base I need to create proper UE4-like normal decals and some other stuff, but those are the easy part. What I need to figure out right now is how can I get that content from RTs. I'm unable to find anything about this in the documentation, but this should be possible - after all, Unity already offers a way to fetch values like linear eye depth, so asking to fetch something like smoothness is not exactly an exotic thing.

Can anyone help me out?