kepub.epub with epub 3.0
Edit: I just found that the KT does not care whether it is epub 2.0 or 3.0. The important part is in the css
Code:
html {-epub-writing-mode: vertical-rl}
And, of course, the extension kepub.epub.
Be aware that kepub.epub has some side effects (as no highlights or annotations without further manipulation of the html files).