Besides the "official" BML page listed in this community's profile, I was also pointed here for some more BML docs.
Between this, perl (general perl-related discussion), and lj_perl (livejournal-specific perl-related discussion), there's pretty much a community for all the bits of the LiveJournal source now. </shameless advert>
My goal for the next two days is to work my way through update.bml and talkpost_do.bml -- so I may well be back soon. ;-)