On having explicit control over what receives the planar reflection - In a deferred renderer like UE4 you have to fit data like this into the GBuffer, which is pretty much full. Reflections are done in a deferred pass long after materials and components are rendered. The nice thing about this approach is that you can layer different reflection methods easily, each one is just a different pass that blends into the reflection buffer mapped onto the screen. For example if the planar reflection is faded out, SSR kicks in, and if SSR is faded out, reflection captures cover the reflection, and they are blended between.
So I don’t intend to add direct control for what receives planar reflections at this point. In theory, planar reflections are accurate for a slab of positions near the plane, there’s no reason you should have to mask them to certain objects. Same reason we don’t have flags for each reflection method (SSR, reflection captures) on receiving components / materials. Instead I want to improve the heuristics for when the planar reflection is valid or not, so that it ‘just works’.