I've been tinkering with this - just doing the change of extension to .kepub.epub and then tweaking the contents.opf to add the cover-image property as described in post #15.
It kind of works for me, thought the cover page doesn't always display properly - I have to turn one page forward then one back. I haave a hidden heading in this page so that it gets added to the toc, and when displayed as a kepub I think the viewing are isn't quite as large, so it somehow gets the display wrong.(if that makes any sense).
I'm using a Kobo Mini with fw 2.3.1. My image is originally 600x800 but the cover page is coded to display it as 548 x 760 using the magic incantation for svg tags I saw posted elsewhere on this forum.
the real win comes with footnotes - the book I have has these with superscripted links to endnotes at the end of each chapter. In epub form, it is difficult even to get the link to work, but in kepub, they work pretty reliably and it gives you a popup preview (usually big enough to see the whole footnote).
Touching on 'see more' takes you to the end of the chapter to see to footnote proper. The 'back' links work correctly as well, taking you to precisely where the original link was, rather than some random point prior to that when in epub mode.
So for a book with footnotes, this is definitely worth doing.
It would be nice to get the cover image working seamlessly though.
It does display OK on the 'home' page as well as in the library, so
I'm guessing it has been parsed into the .kobo directory, I guess the
problem might lie in my cover page html or the image resizing.