Did Adi Shankaracharya (directly) mention anywhere in his writings, Saraswati as Para-Brahman?
Yes, in his (Sankaracharya's) famous hymn Saundarya-Lahari. In verse 97 it says (Swami Tapasyananda translator):
O Consort of Parabrahman! The scholars who know the real meaning of the Agamas (scriptures) describe Thee as Sarasvati, the Goddess of Learning and the Consort of Brahma. Besides they speak of Thee as the lotus-born Laksmi, the Consort of Visnu, as also as the Daughter of the Mountain (Paravati) and the Consort of Hara (Siva). But Thou art however the Fourth (the unique power that is the source of the three deities mentioned) of inconceivable and limitless majesties--the indeterminable Mahamaya who revolves the wheel of the world.
[Swami Tapasyananda's note to this verse] Notes: By describing the Devi as the Fourth (Turiya) and the Consort of Parabrahman (Parabrahma-mahisi), it is indicated that the Devi invoked here is the uninvolved and unaffected Power and Substance that has actually become manifestated as conscious and unconscious principles, including divinities like Sarasvati, Sri, Parvati, etc. How She can at the same time be uninvolved and unaffected and yet becomes all these manifestations is a mystery impossible for the human mind to conceive. So She is described as Maha-Maya (the great and mysterious Power) and duradhi-gamana-nissima-mahima (one of unfathomable and limitless greatness).