Found this interesting project by Christopher Deckers, SWTSwing, aimed to port SWT to Swing to bring portability to SWT. It goes beyond that, filling the gaps in each world. You can use a SWT widget that is not available in Swing and vice-versa.
On the Windows port, there is support for OLE: SWT can offer particular native services to the developer. That means in the Swing port there could be an access to the Swing objects. The developer could then use the best of both worlds.
Since Swing components are used by the SWT implementation, there could be the possibility to include any kind of swing components in a specific Swing container. And also provide a JComponent meant to contain SWT widgets. This would allow developers to be able to deliver applications when a component is not available for the graphical library in use but exist for the other one.