The current configuration of the CompositeClassLoader cannot find classes in OSGi bundles that depend on the bundle that loaded the XStream JAR, since it only looks there in the bootstrap loader. With a line of code like this
((CompositeClassLoader) this.stream.getClassLoader()).add( getClass().getClassLoader() );
I was able to work around the issue, but I would like to suggest a solution that does it out of the box:
Method Annotations.configureAliases() (or something it calls) could add the ClassLoaders of the configured classes to the CompositeClassLoader, thereby ensuring that they can be loaded later.