XML was an aim for LyX 2.x, but the devel wiki only mentions it briefly. I didn't find any follow-up.
I offer "Docbook 4.4 XML" as a more specific target, in case there are no better suggestions. Why? O'Reilly publishers now recommend Docbook 4.4 XML to their authors (cf http://oreilly.com/oreilly/author/ch02.html and scroll to the section heading "Tools"). You'll see that: * O'Reilly now promote proprietary tools for editing; * they have a "primitive and less supported" stylesheet for OpenOffice, which hasn't been updated for 4 or 5 years, and * they no longer recommend LaTeX. If LyX had a reference on that page as a recommended editor for Docbook XML, it would boost the user base (and it would give authors more choice for open source editing). The DTDs for Docbook 4.4 XML are at an OASIS site, http://www.docbook.org/xml/4.4/ according to the O'Reilly page referenced above. Regards, Douglas Ray