You can’t do this with a single class, unfortunately.
To be referenced, the class has to inherit from CBLModel. But CBLModel expects that it owns a document, so it can’t be used for embedded objects.
This would be a good use case for multiple inheritance, if Swift (or Obj-C) supported it. As it is, you’ll have to make two different classes. You could define an interface that defines the properties, and then have both classes implement it — this will make sure the properties stay consistent between the two.