Trying to delete an Unpublished Instance fails

Summary: It shows the error "That path does not point to a Thing that you are allowed to see".

Eric originally reported this in a more-specific form related to sub-Models, but the problem applies to all Unpublished Instances. The problem is pretty obvious in the code: SpaceCore.DeleteThing doesn't do all the song-and-dance to route through to PublicationCore that ChangeProps does.