I just came across a rather neat little trick over on MobileRead. It turns out that the ODT file format (used by Open Office) is virtually identical very similar to an Epub file. No, seriously, it is.
Change the suffix of an ODT file to ZIP and you can open it like any other ZIP file. If you look inside you'll see most of the components of an Epub.
It's missing parts and for obvious reasons it doesn't validate. But I was able read the Epub file in NookStudy (without difficulty). I then tried to fix the problems with Sigil, the Epub editor. Unfortunately Sigil couldn't load it. I don't blame it; this is a kludgy way of making an Epub.
This might not amount to much more than a trick, but I think it's shiny. But given that there's an Epub maker plug-in for Open Office, it really only is a trick. It's not really good for much. You need OO to make an ODT file, and with the plug-in you might as well make a valid Epub.