I am not aware of any such documents.
As a PhD student, you know that nothing beats looking at the actual code. WindowBuilder is now an Eclipse open source project, so you can check out the WB source here...
You should start by looking at the org.eclipse.wb.core.parseFactories
extension point and its various implementors like org.eclipse.wb.internal.swing.parser.ParseFactory
Take notes on what you find, write it up, and contribute it back to the project. We could always use more docs (especially on the internals).