I created a workaround build configuration that will create aggregated javadocs from generated sources, although the call sequence has two steps:
mvn package
mvn -N pre-site
The build configuration is now published in Github. The current version only supports a project tree of depth one, but can of course be modified. It works by gathering the dependencies under the parents target directory and then running the included Ant script.
Finally, if running under Jenkins, the mvn -N pre-site can be invoked in the same job through Execute shell post step. Publishing the javadocs in our version of Jenkins required using the post-build action “Use publishers from another project“.