It is possible to overlay WEB-inf/lib/xy.jar without having xy.jar included in the war file. If an EAR file is used a module in the lib directory can not be added in the same way. But overlay is working if the lib/xy.jar is available.
This impact not only EARs.
https://github.com/jbossas/jboss-eap/pull/1699
Verified in EAP 6.4.0.DR3.