There should be an @Ext annotation which can be used to specify an externalizer class for another class. Kinda like this:

@Ext(TheExternalizer.class)
public class MyNotSerializableClass {
...
public static final class TheExternalizer implements Externalizer

{ // nested classes can see private members of their enclosing class }

}

@Ext would only accept class arguments that extend Externalizer.