As of: https://github.com/jbossas/jboss-as/commit/38815193011fba624170a7d938118ee630b6ccb2 it is possible to deploy persistence.xml from invalid location:
As per JPA spec(https://docs.jboss.org/author/display/AS71/JPA+Reference+Guide), valid locations are:

• an EJB-JAR file
• the WEB-INF/classes directory of a WAR file
• a jar file in the WEB-INF/lib directory of a WAR file
• a jar file in the EAR library directory
• an application client jar file