Is it me or is it odd, that a reader called "Hyphen" doesn't seem to offer hyphenation? Especially as the screenshot shown on the page shows rather large spacing used for block setting the text.
I like the Knuth/Plass technique as well, it creates much more uniform and visually appeasing line margins. The reason something like it would be so difficult to implement is that it requires heavy geometric processing every time the content reflows, which would impact Hyphen's performance a lot. Another issue I see is that it would pollute an ePub's DOM more than I'd care for.