The author who quoted RHS, that LB referenced in the OP, wrote:
“I saw R. Schachter and asked him about this matter. He reaffirmed his position, stating that whom one marries is a halakhic matter and therefore a posek can indeed tell you whom you must marry. He added that this is almost always theoretical since in order to make such a ruling the posek would need to know both the bride and groom for many years so as to be sure that what he is saying is correct. But he also insisted that if the posek does have the requisite knowledge he can indeed give a binding pesak about whom one must marry.”