It is always worth taking seriously the views expressed by Abbot Rupert of Deutz (ob ca 1130) with regard to the reasons behind the liturgical choices made in our present (S Gregory I/S Pius V) liturgical books. So I offer to your learned consuderation his views on the Transgfiguration Gospel shared by the Saturday Ember Mass and the Sunday which follows it.
"Priests [sacerdotes] having been ordained on Saturday, and the other ministers of Christ's Altar, this Gospel is fitly read ... For priests and the other ministers of the Altar, when they are promoted to so great a ministry, as it were ascend with the Lord up a very lofty mountain, that with no veil [revelata facie] they may gaze upon his glory, so fit to be contemplated, and might understand what with him or concerning him Moses and Elias, that is, the Law and the Prophets, are saying, and may proclaim it to the people below [inferiori], and having with the faith of Christ the morality which comes from the Law and the doctrine which comes from the Prophets, they may be ready to give an account to everyone who seeks it ..."
Expositors sometimes offer clever and elegant accounts of why we hear the Transfiguration at the start of every Lent. I feel pretty confident that Abbot Rupert gives us the real reason for the choice of this pericope in the Authentic Form of the Roman Rite.
Archbishop Myers remarked upon how many opportunities are offered for Ordination in the Lent of the Authentic Form of the Roman Rite. I suggest a pragmatic reason for this ... all the actuosa participatio in the rites of Holy Week and Pascha required a full complement of clerical functionaries!
This stuff is sacerdotalist, even clericalist, and unashamedly so. Priesthood is an august mystery, an intimate association with the Lord himself. (Rupert's own priesting, incidentally, was delayed because of his refusal to accept Holy Orders from an excommunicated bishop.)
The imagery of going up and apart with the Lord to behold his glory, may have been the more striking to priests and people alike when (visually) it meant going with the Domnus Papa beyond the great curtains or hangings which surrounded the Altar. As one of the Ordines puts it, the Pontiff "intrat in Canonem".