This is an older message, but if you are looking for the same information, our TAM told me about an (until then) undocumented option on the conversation object:
There is an API object for the unread count for threads, it’s called unread_reply_count and it’s called as part of the conversation collection.
e.g.: select conversation.unread_reply_count from messages where id = '1111'.
It returns 0 for threads with no replies, so you still need to use user_context.read in some way.
Hope this helps the next person searching for an answer to how to do unread counts on threads in message lists.
