There's currently no way to set the labelFor property in the property inspector or anywhere else in the Scene Builder.

Perhaps related is that the scene builder is unable to cope with labelFor="${foo}" references, but labelFor="$foo" references do "work" as in they don't give warnings. Since the latter notation requires that the references are back-references due to single-pass parsing of the FXML file, it would be nice if the Scene Builder supported the ${foo} notation.