It is the same for referencing a library from a har as it is a sar or ejb jar.   Stick it all in your ear...
EAR LayoutCode:
   me.ear
     +- META-INF
     |   +- application.xml
     |   +- jboss-app.xml
     +-lib
     |   +- hbm-data.jar
     +-me.har
         +- META-INF
            +- MANIFEST.MF
            +- hibernate-service.xml
            +- left.hbm.xml
            +- right.hbm.xml
jboss-app.xmlCode:
<?xml version='1.0' encoding='UTF-8' ?>
<!DOCTYPE jboss-app
    PUBLIC "-//JBoss//DTD J2EE Application 1.3V2//EN"
    "http://www.jboss.org/j2ee/dtd/jboss-app_3_2.dtd">
<jboss-app>
  <module>
    <har>me.har</har>
  </module>
   
</jboss-app>
HAR manifestCode:
Manifest-Version: 1.0
Created-By: Buddha
Class-Path: lib/hbm-data.jar