Reference Image in the Prefab Editing Environment

When building UI, I usually like to add a full screen reference image to build with. Unfortunately, with the the new prefab workflow I would have to edit my prefab editing environment scene every time I want to use a reference image, since you can't add anything the editing environment scene while in edit mode.

Could future versions of the prefab environment allow us to add items to the Prefab Editing Environment and not just the prefab we're editing?