since kompozer already support xhtml, xml, and css as well. How if it also support .epub editing. It just of editing xhtml, opf, and ncx (2 latest is xml files saved with different extensions). And put it all together in zip file (saved as .epub).
The kompozer should have :
1. xhtml splitter. It can split xhtml with all necessary header and footer copied on result file.
2. xhtml combine. The necessary header and footer of second file will follow the first file.
3. xml schema for opf and ncx file following epub specification.
4. put addons for epub validator. .epub guys already create the software, it just need to be called from kompozer