I feel a bit like poor Mouse today. It's taken me three days and I have a whopping headache, but I've created an ePub from scratch - all the HTML and CSS files, and all the other ones that I'd never heard of before like content.odp. I had to download several programs so I could zip it up and then check that the code was valid. Many, many errors later, I have clean files that pass validation and I'm hoping I now have a repeatable process.
If anyone wants to try it, I've linked to some of the resources I used in a post on StarBytes Press.