I've spend a week or so to come up with a solution for this for v2, but in the end it wasn't useful. The main issue is that the project isn't a hierarchy of data, it's an object graph with a lot of object references.
So I ended up with an XML format which contained a lot of reference tags to elements which were defined elsewhere to illustrate a reference. While this would work, it doesn't help one bit because it would still be a large XML file with a lot of reference specifications which then can't be versioned as well because 1 change can make the whole XML data invalid.
So this lead to the conclusion that it wasn't really possible to create a solid xml file with solely data which was versionable.