From patch notes:
You can now query for a list of completed quests with "QueryQuestsCompleted()" then wait for the "QUEST_QUERY_COMPLETE" event, and call "etQuestsCompleted(<luatable>)"
I was not holding my breath on that, but glad to see Blizzard added it. I will definitely try to support it in the version of Carbonite that comes out for patch 3.3.