One of the Lightroom engineers (LR_Kevin) on the Adobe feedback forum: "You will be able to create your own templates at any size you like, so a printer could publish a set of templates sized perfectly for them. You would install the templates, print your book to PDF, and upload the PDF file to the vendor directly. I'm working on documentation for template creation, but it needs a little more smoothing out before releasing it to the wild. :-)"
So no guarantee when, but at least it's coming!