Post processing blendable before lighting pass?

Since UE4 uses deferred rendering it seems like it should be possible to alter the G-Buffers for color, normals, metallic, roughness etc, before the lighting pass. I mean in a way such that I can look at the neighbouring pixels, so a normal material won’t work. Is this possible?

This would be optimal for me too. I have lot’s of optional gbuffer modifications which give either constant performance drop or shader permutation hell. Good use cases are wetness mofication, frostiness, procedural dust/wear, analytical roughness modification to prevent specular aliasing. And lot’s of others.