Discussion on the EclipseLink bug https://bugs.eclipse.org/bugs/show_bug.cgi?id=383200 ("On JBoss AS 7, EclipseLink doesn't find entity classes unless they're explicitly listed in persistence.xml") suggests that JBoss AS 7 may not be providing a spec-compliant PersistenceUnitRootURL or contents.
EclipseLink can't scan the contents of a deployment for @Entity annotated classes, so it doesn't find any entities unless they're explicitly named in persistence.xml .
Returns the URL for the jar file or directory that is the root of the persistence unit. (If the persistence unit is rooted in the WEB-INF/classes directory, this will be the URL of that directory.) The URL will either be a file: URL referring to a jar file or referring to a directory that contains an exploded jar file, or some other URL from which an InputStream in jar format can be obtained.
Returns: a URL referring to a jar file or directory
... and that's what EclipseLink expects. However, for EclipseLink class scanning to function correctly an adapter class must be provided, such as the classes provided here:
to integrate EclipseLink with JBoss's VFS.
It may be worth investigating why this is necessary and whether the current behavior is standards compliant.