The history of changes to collection membership can be treated as a property of the collection. The mapping of references to their targets could also just be in server someplace, not a property on a referential resource. There needs to be a way to discover and control linkages. Clients need to be able to find out which things in a collection are references and which have their own content. Keeping references as independent resources is better than changing the nature and semantics of collections, which would require changes to the WebDAV specification.
References are resources.