This sounds like a good idea, Stefan. I don't know if any tools do this, but the best UML to Java tool is Together from togethersoft.com. I think they have a free evaulation version? You might give it a try.
If you're up to it, join in the development of ArgoUML at
http://argouml.tigris.org/ and propose adding an Export to XML feature. I'm sure it would be well-received, especially because the XML would be more language-independent, and could support C++, C#, Ruby, and others languages as well, and could leave room for various XML-to-java optimizing parsers.
Let me know if you did find or do find a UML-to-XML tool - I'd certainly be interested!