I'm the lead developer of Antenna House Formatter, a print formatter supporting international text layout with W3C's specifications including XSL-FO, CSS3 Paged Media, Writing Modes, Text, etc.
We have participated in the work for the JLREQ (Requirements for Japanese Text Layout, W3C Note) and that document is referenced in CSS3 Text and other specifications.
Making books, both electronic and print, is also our interest. We have book authoring service CAS-UB (Cloud Authoring Service for the Universal Book).
Paged Media layout specifications will be very important for eBooks and publishers.
EPUB and Print (PDF) books can be produced simultaneously from one source with each set of style sheets, for EPUB readers and for print formatter. Our CAS-UB is an example of such production system, and another good example is the workflow at O'Reilly. In this case, we use the Paged Media layout spec for print.
I believe that future EPUB readers will have rich Paged Media layout features.
The JLREQ document is valuable not only for Text, Writing Modes and Ruby, but also for Paged Media related specifications. For example, page floats and footnotes in both horizontal and vertical writing modes are explained in the JLREQ (Chapter 4: Positioning of Headings, Notes, Illustrations, Tables and Paragraphs). We have implemented such features using this document as reference.
I expect that the international text and page layout will be discussed deeply in the Workshop and I would like to participate there.