I had a chance to do a little fiddling around.
The vast majority of the code is identical, but there are a few differences (if it's not shown below, it's the same as standard epub or Mobi):
CONTENT.OPF
In metadata:
<meta name="primary-writing-mode" content="vertical-rl" />
In spine:
<spine page-progression-direction="rtl" toc="ncx">
<itemref idref="x_p-cover" linear="no" properties="page-spread-left"/>
(and obviously more lines for the other XHTML files)
CSS file:
html,
.hltr {
-webkit-writing-mode: horizontal-tb;
-webkit-writing-mode: horizontal-tb;
}
.vrtl {
-webkit-writing-mode: vertical-rl;
-webkit-writing-mode: vertical-rl;
}
And that's it!
Everything else is pretty obvious, and applies to both horizontal and vertical display, as far as I can see.
However, there are hundreds of entries for handling gaiji, grafix, lines etc vertically. Since they are unique to vertical text, I assume the lines above are really the only ones that matter. Font definitions and body text <p> definitions are simple, and essentially identical to horizontal layouts.
Next time I have some spare time I'm going to take a random chunk of Japanese text and build an ebook around it using the above code, just to see what happens.
I'll post the results here.
The Japanese ebooks I dissected don't show the generator in the metadata list.