At the moment there is no DCD or XML Schema... The closest thing might
be the Scripting API documentation (which is not even close really)
because it outlines the object model which translates (more or less)
into the structure of the XML written and read by ExportXML.js and ImportXML.js.

Those scripts are just sample scripts that are meant to provide a
boost for people who want to do various forms of project interchange.
They were originally written mainly for testing purposes and with the
goal of being low maintenance through the use of reflection... in
most cases, if the scripting object model changes, so will the
XML. They were not meant to define a standard for XML import and
export of Vegas project data.

I suggest that people who are doing project interchange take those
scripts and, if needed, incorporate them into their project making
changes as needed. But more likely, such people would just want to
take ideas from those scripts in building their own... XML may or may
not even be used for any given project interchange tasks.