I’ve ended up talking to my former PhD student about the pileus quite a bit over the past year. I’m creating this post to have place to store references.
Livy 38.55; 187BCE: Ser. Sulpicius next consulted the senate as to who was to conduct the inquiry, and they fixed upon Q. Terentius Culleo. There are some writers who assert that this praetor was so attached to the family of the Cornelii that at the funeral – they say he died and was buried in Rome – he preceded the bier wearing a cap of liberty, just as though he were marching in a triumphal procession, and at the Porta Capena he distributed wine sweetened with honey to those who followed the body, because amongst the other captives in Africa he had been delivered by Scipio.
Plutarch, Numa 7.5: Now before this time the Romans called their priests “flamines,” from the close-fitting “piloi,” or caps, which they wear upon their heads, and which have the longer name of “pilamenai,” as we are told, there being more Greek words mingled with the Latin at that time than now.