Setting aside the rules to favor preference (the case if *Manospondylus gigas* was ever validly diagnosable) would fly in the face of those rules being purported as "good" enough for said scientists to appeal to.
But, you know, the ICZN explicitly provides the possibility of petitioning the Commission for the maintenance of such widely used names as *Tyrannosaurus*.
It is in cases like this that I would tend to favor usage of *Rioarribasaurus colberti* for the Ghost Ranch taxon _as long as_ the holotype of *Coelophysis bauri* was undifferentiable from at least two other taxa. (And let's give this "rule" its due, it really should be worded that way on the case of diagnosis -- I think Mickey was the first to formulate this argument, correct me if I am wrong.)
For nomina dubia, Article 75.5 _explicitly_ says people "may" petition the Commission for replacing the undiagnostic type with a diagnostic neotype. http://www.nhm.ac.uk/hosted-sites/iczn/code/index.jsp?article=75&nfv= This is why the petition to give *Coelophysis* a neotype was accepted. (The article even provides an example where the name of an ammonite species was "saved" this way.)