In addition to @ValidateReference, if a validator tries to validate a property and that property has a @ValidateReference to some other class, and that other class also contains an @ValidateReference to another class that has constraint annotations, I'd expect the validator to actively persue each @ValidateReference (recursively?) until it finds the terminal constraint annotations that will actually validate the values.

The AnnotationProcessor (used when compiling Java files) should probably give warnings of an @ValidateReference is referencing another @ValidateReference.