How to get alpha channel in frame's texture

I have a default VR project.
I want to pixels representing sky to have RGBA value of (0,0,0,1). How do I do that, preferably in the editor?

Context: I have a custom VR plugin that works. I want to add AR functionality to it.
In my plug-in code, I’m looking at viewport’s texture’s memory but I’m not seeing alpha channel there. If I set the sky to be black and transparent I get (0,0,0,0) pixels, not (0,0,0,1) like would expect.

I get viewport’s texture in
FXRRenderBridge::UpdateViewport(const class FViewport& Viewport, class FRHIViewport* InViewportRHI)

I’m very new to UE4; does my question make sense or do I need to phrase it better?

(edited; whoopsie)
I don’t think alpha is supported on post process materials, yet. But checking for scene depth should give you a mask to remove the sky.

Another edit (I should get more sleep): no need to 1-x if you want the sky to have a value of 1.