How can I get GameObject.FindObjectsWithTag() to work in Prefab editor

I've got some [ExecuteInEditMode] scripts that would really be helpful in new nested prefabs I'm making. They work fine in any scenes I'm editing, but when I've opened a prefab in the new prefab editor and I try and use this script, it only returns objects that are in the currently open scene behind the prefab editor. Editing a prefab does not "open" it, like a editing a scene does, instead it just displays its information over the top of everything else, hiding the fact that the scene you last had open is still open.

Can anyone help?