Tasks and age of Military Tribunes during the Late Republic and Principate

[Nathan Ross]

having embarked upon the senatorial cursus honorum, how could he serve as an equestrian tribune?

Ah yes, you're surely right. I was confused by Pliny becoming sevir equitum romanorum after his tribunate, but it seems that other holders of this honorary office were bound for entrance to the senate. So although he's nowhere actually described as laticlavius (as far as I know) we can probably assume this was his position.