There is a port of SWT and JFace called DWT written in the D programming language, it would be really great if SWT Designer could add support for it. Java and D are so similar (in this case at least) so I think it would be quite easy to add support for it.
The public API is almost unchanged, the biggest differences are: instead of "org.eclipse.swt" it's "dwt" and instead of "SWT" it's "DWT". There are a couple of small changes between Java and D also, I can list them here if you want to. I don't know exactly what the D plugin needs to be able to do but it has: Syntax highlighting, formatter, Syntax errors and some semantic errors, Autocompletion, Go to definition and a lot of other features so I guess that means it can build an AST model.
If SWT Designer was open source I would try to modify it myself to output D code.