PDE adds plugins to the Java Search engine so that it creates a special java project (External Plug-in Libraries) containing a separate classpath container (ExternalJavaSearchClasspathContainer) that includes classes from those plugins that are defined by the user or plugins from the target platform. The project isn't visible in the Package Explorer by default.
We could use a similar technique. I suppose, it wouldn't be so difficult to add the Ctrl+Shift+T (Open Type) functionality to the Source Lookup plugin.