Hopefully I'll get to comments sync over the weekend. I wrote it once in O'Caml as an exercise.
Due to quirks of LiveJournal history, the entry sync and the comments sync are entirely separate and almost diametrically opposite protocols -- the entry sync uses challenge/response for authentication, a flat protocol for transfer, and holds all the goods in one request, while the comments sync uses an LJ cookie, an REST/XML-based protocol, and has half the data in one request and half in the other.