All the customizations you have seen so far were based on modification of a generated stub class with accompanying generated Guice bindings in the module under the
However, since Xtext relies on injection everywhere, it is possible to inject a custom implementation for any mechanism, even if no stub class has been generated.
If you installed Xtext SDK in your Eclipse, the sources of Xtext are available for you to inspect. You should learn to inspect these sources by navigating to them and see what gets injected and how it is used. Then you are ready to provide a custom implementation and inject it. You can use the Eclipse Navigate menu. In particular, to quickly open a Java file (even from a library if ...