Many thanks to site commenter Ross for providing an update about the status of Book 12 (Beneath the Tree of Heaven) and Book 13 (Song of the Bronze Statue). He writes that David Wingrove let him know:
Hey Ross, the cover’s just in. Book is typeset. So going into production next week. Sorry for the delay. I’m hoping book 13 will be following not too long afterwards. Cheers, David
Hopefully this is good news for those of us who have been waiting for any news since the publication of Upon a Wheel of Fire in November of last year.