The <extension> element in standalone.xml is not meant to be a generic facility for loading ServiceLoader-based services. It's meant for installing implementations of org.jboss.as.controller.Extension (i.e. new manageable subsystems) and nothing else. That's why your dummy implementations get loaded.
If you're not looking to add a manageable subsystem, I recommend deploying a sar with a META-INF/jboss-service.xml that declares an mbean whose start() method does the ServiceLoader.load() call.