Again, maybe I'm too late, but: Is a switch to XML is really a good idea? The XML hype is over, we don't use Java, LyX doesn't run on a Application-Server, and we live now in a App/web-app world where Json is more common than XML.
And living in the Latex world this means we should not use XML but the equivalent for Json in Tex: Lua. http://www.lua.org/ In future there is a Latex with an embedded script language http://www.luatex.org/ which we could use NOW for our needs. We could use Lua internally in LyX as scripting languange (footprint: 100kB) and as file format similar to JSON: In a LyX document are function calls which are are bound to C++ functions. Loading a document is simply done by setting up the binding and to execute the script. Similar to SAX with XML, but much more readable and faster (for x86 there is a Just-In-Time compiler). Sure, we could export to some XML format, but we should not make XML the first-level LyX file format. Peter HTML book: http://www.lua.org/pil/
