So I started a while ago looking at the OpenOfficeXML specification (5000+ pages...) and, given the very low quality of the HTML export of Microsoft Word, decided to write some code to convert OOXML documents into clean HTML+CSS documents. It's far from ready but the basic architecture for my converter is now done and it already works for trivial documents. When it's ready (this is not for tomorrow, it's going to take some time...), the code will be integrated into BlueGriffon EPUB Edition and sold as an add-on to BlueGriffon.

Here is a little video I just made, you can also watch it on YouTube directly.