Unity 5.6.1f1 Odin 220.127.116.11
If a class is derived from SerializedScriptableObject and implements an interface, it will not show up in the selector for a interface field. I updated the code in your SerializeAnything.cs example script. You will see the class called problem class. In the image below you can see problem class is not one of the options that is selectable. Only ImpA and ImpB are selectable.