I've wanted something like this for going on two years. I even went so far as to start building it (wireframe: http://basseq.com/fun/epub/; some epub publishing code written), too, but lost momentum back in September and it's been sitting in my project folder.

So I'm excited someone took this further. I'll be playing with this more. Drop me a line if you're looking for more thoughts, feature ideas, or help.