The 'Rubrical Directory' to our Missal (Paragraph 11) provides that "another cleric or even an instituted Acolyte may serve the subdiaconal ministry and read the Epistle".
Nice to have this splendid encouragement for restoring the proper old High Mass with the three Sacred Ministers walking one behind another up the church "just like a train", as one very approving girl at S Thomas's commented.
Could be the first mention of Hypodiakonia in a Roman document since the 1960s?
Priests are clerics: so a spare priest could "Do subdeacon". How delightfully flexible we are in the Ordinariate. Since this is possible, ex analogia a presbyter could also deacon.