@fxp33: In case you want to generate backwards-compatible text, Ryan Stenhouse, came up with a
vertical text hack, that
works in Sigil, doesn't require <br /> tags after each Kanji and even supports vertical mono-spaced Latin text.
BTW, when Amazon started selling their Paperwhites also in Japan, they added vertical text support to new models and updated the firmware of older Paperwhites to support vertical text. For more information see this
blog post.
The Kindle Publishing Guidlines mention that the Kindle compiler partially supports Sharp's
XMDF format; does the Kobo support this format, too?