A couple other comments:
There's a split package; i.e. org.jboss.modules appears in embedded-modular-bootstrap. Split packages are verbotten, so we can't push this upstream until that's fixed. Hopefully there's no reason ModuleXmlParser can't be made public.
ModularEmbeddedServer.start(String... dependencies) throws an NPE if you pass in null dependencies.
The need to pass in those dependencies indicates StandaloneServer has a problem that needs fixing. Embedded users should be able to get a simple notification when the server is "started", which I'd define as the point where we log the "JBoss AS %s \"%s\" started in %dms" message.