@edvin I recall being able to combining -at-FXML annotations in a toplevel controller from included FXML elements but can't seem to get this to work. I don't have the time to investigate this hunch right now. I think this modality might be good for TornadoFX since the users won't be specifying a Controller class with each FXML. That is, the fxid() constructor would flatten all the fx:included files' ids.
The test case I added has an impractical Scene graph navigation to validate that the Nodes were created correctly, but no one will want to program like that to get at included contents.