The global lib directory is for the bootstrap libs needed to start JBoss. If you want to have a common lib directory that is shared by all configurations then add the same common directory path to the URLDeployer mbean in jboss-service.xml in the conf directory of each configuration.
For example: Add a mylibs directory under the Jboss root. Put you libs there. Add the following entry to jboss-service.xml in the URLDeployer mbean near the bottom of the file. There should a URLs attribute set to ./Deploy. Add , and then ../../mylibs. This will cause your libs to be deployed. Remember to do this for each configuration.
That does help.
For my education: Classes in jars the global directory are not visible to deployed apps? Tell me if I'm wrong here, but JBoss does not actually deploy the jars in the global lib directory for use by the rest of the running server. It *only* uses them for bootstrap?