Defining classes of quoted triples in RDF-star/RDF 1.2

Question

RDF-star (soon to be RDF 1.2) introduces a new term, quoted triples, that can be the subject or object of a triple. In the Turtle 1.2 syntax, an example of a basic quoted triple would be written:

<<  <IRI1>  <IRI2>  <IRI3>  >>

Now suppose that I want to have the range of a property include not just a specific quoted triple but a class containing certain kinds of quoted triples. In particular, the quoted triples I want to allow in the range of the property are of the form:

<<  (instance of class C)  (predicate P)  (instance of class C)  >>

How does one define the class of quoted triples of this form in RDF 1.2/RDF-star, so that one can say that the range of a property includes instances of that class?

I tried using something like the following:

<predicate1>
  rdf:type  owl:ObjectProperty;
  rdfs:range  [ 
                rdf:type  owl:Class;
                [
                 << [rdf:type <classC>] <predicateP> [rdf:type <classC>] >>
                ]
              ].

However, I don't think that what follows owl:class is even valid Turtle 1.2, much less that it is semantically correct. (I am also not sure if there are reliable Turtle 1.2/Turtle-star validators, so recommendations there are also appreciated.)