From a reader…
Would it be licit for a Bishop to offer to serve as Deacon for a Extraordinary Form Solemn High Mass Extraordinary Form celebrated by a Priest who is not a Bishop?
That would be just plain weird.
I have no idea how the ceremonial of it all would play out. Fortescue, Wapelhorst, … any of the other standard Rubricians wouldn’t be of much help to figure it out.
This is in the great category of The Possible. A Bishop was ordained as a Deacon. He still possesses the diaconal character.
It is also in the great category of The Inadvisable.
It would be like … putting ketchup on sushi or playing Vivaldi on a banjo.