After checking out the
Abiword 3.0 ePub plugin, I decided to give
Calligra Author 2.7.5 a whirl with Kovid Goyal's
demo.docx.
The HTML output was much better than the Abiword 3.0 output, but the output file was also invalid. Since Calligra Author can also generate Mobi7 files without Kindlegen, I also generated a .mobi file for completeness' sake. However, this file, too, was invalid and could only be opened with Calibre.