This post summarizes what I want to do for the present 4.5-year period. And looking at that again, I'm going to answer my own question 
(but you can agree or dis- with the answer).
I think Tranquillina, in character, would be writing a bunch of Intrigue-type letters all the time - one per season or possibly even more - as she keeps up correspondences with magi she's met all over the place, not least back in Stonehenge. So the people listed in that list are already sort of exemplars for this enterprise. Thus, perhaps using just one of them - Ariel, I'm tempted - as an exemplar for the whole batch might make sense.
So maybe I could write one letter to each of the following: Ariel (representing the whole Intrigue batch), Thamik, Aurulentus, [strike]Goliard[/strike], Findabair, and Ra'am for Magic Theory. Although Ra'am is on the Reserved Personae list, so we should think about that: mechanically, I could simply choose someone else to write about Magic Theory; but in character, it's a good relationship to cultivate (and Tranquillina is about to answer Tria's question about that very thing!).
Edited to add: Now that our Parma Magica book is only Quality 12 instead of Quality 17, I don't really want to do Parma correspondence for the time being (since I won't round up to the next Ability level any more). I can reallocate those three seasons to one each of Corpus, Magic Theory, and Intrigue correspondence, I suppose.