We wouldn't use the term "Mediatrix of All Graces", but I know that Saint Gregory Palamas believed that Our Lady distributed the "eternal riches of the Godhead." I was told by a very holy hieromonk that Saint Gregory's words about the Mother of God were true and not mere hyperbole.
From St. Gregory's homily on Our Lady's Dormition:
"She alone has received the all-pervading fulness of Him that filleth all things, and through her all may now contain it, for she dispenses it according to the power of each, in proportion and to the degree of the purity of each. Hence she is the treasury and overseer of the riches of the Godhead. For it is an everlasting ordinance in the heavens that the inferior partake of what lies beyond being, by the mediation of the superior, and the Virgin Mother is incomparably superior to all. It is through her that as many as partake of God do partake, and as many as know God understand her to be the enclosure of the Uncontainable One, and as many as hymn God praise her together with Him. She is the cause of what came before her, the champion of what came after her and the agent of things eternal. She is the substance of the prophets, the principle of the apostles, the firm foundation of the martyrs and the premise of the teachers of the Church . She is the glory of those upon earth, the joy of celestial beings, the adornment of all creation. She is the beginning and the source and root of unutterable good things; she is the summit and consummation of everything holy." http://www.pravoslavie.ru/english/38894.htm