Writing a Swixml declaration file is not that hard, if you know your javax.swing components. However, getting a little support from your XML-Editor or IDE certainly doesn't hurt.
Getting IntelliSense help sometimes doesn't require much more than pointing your editor to a DTD or XML Schema to pop-up element and attribute names as needed; or simply adding an xsi:schemaLocation attribute to the root element might do the trick.
Anyway, I've generated the XML-Schema for all built-in Swixml elements and attributes and made it available here: http://www.swixml.org/2007/swixml.xsd
It is complete, concerning elements and their attributes, and while the schema already has more than 2700 lines (which tells a story about the complexity of Swing), it does not include containment-rules, e.g. that you shouldn't add a button into a label, nor does it validate the attribute values other than booleans.