From a reader…
I’ve been to Solemn High Masses where a seminarian (perhaps having been instituted as an acolyte) was acting as Subdeacon (a “straw” subdeacon, I think, since he’s not ordained). Other than ordained men, who can act as the subdeacon in this way?
Yes, there can be a “straw subdeacon”. As a matter of fact we had one this morning: an officially installed acolyte took the role. There were slight adjustments to the rite as a result.
There is support for this. The Pontifical Commission “Ecclesia Dei“ once responded to a question posed by the Australian Ecclesia Dei Society in the early nineties about this very issued. The reference for this PCED letter is 7 June 1993, Prot. 24/92 to the Australian Ecclesia Dei Society. The PCED said then that, yes, an officially installed acolyte could take the role of the subdeacon in a Solemn Mass if a cleric wasn’t there to take the role.
Also, as recently as 2013 the Pontifical Commission “Ecclesia Dei” responded to a question about this. They responded, though not in an official public way, that
“[T]his Pontifical Commission would limit itself to saying that the function of Subdeacon can be legitimately assumed by an acolyte suitably instituted by a Bishop, but with the particular appropriate ritual differences.”
What would those “appropriate ritual differences” be? First, they wouldn’t be to the institution of the acolyte, which is how you could read the letter. They concern differences for what the subdeacon does. For example, he doesn’t wear the maniple and the offertory is a little different for his role.