My DH reports...

(1) he's fairly certain it's the seat post because he doesn't hear it when he stands;

(2) when he's sitting in the saddle, the squeaking is worse when he is pushing hard versus spinning lightly;

(3) he had not thought to try carbon paste. He'll pick some up next time he's in Santiago.

I also searched Bike Forums (thanks, Bluebug, for the suggestion) and found a suggestion about getting a longer seat post. My DH is quite tall, and while the seat post is within the maximum range, it might be worth trying a longer post.