I’m not sure that would work in the context of unreal, specially given how deferred decals work
first of all, unreal doesn’t have a way to just render stuff into a separate rendertarget. there’s a few ways (draw a material to render to texture, use a scenecapture actor, use customdepth) but AFAIK none allow you to render stuff from the same view as the camera with the full featureset (i.e. customdepth only renders depth. I guess the closest is to make a scenecapture2d that follows the camera each frame and trick it to render things that are hidden on the view)
secondly, decals can’t be just told to render into a separate rendertarget. if we’d make the scenecapture2d that follows the camera we still wouldn’t be able to tell it to just render decals and nothing else. UE4’s deferred decals are already on a separate pass by themselves, so they aren’t set up in a way that you can render them all without the underlying geometry
I guess with some engine modifications one would be able to access the deferred decals’ buffer to composite it into the scene with some rough Z-testing… but I’m sure unreal is already doing that 