If the application ejb deployment contains the org.jboss.ejb.client.EJBClientInterceptor file it is possible to overlay it.
But if there is no file within the deployed application the overlay does not has an effect.
This is different to i.e ejb-jar.xml where the overlay works nevertheless whether this descriptor is available in the ejb.jar or not.
A reproducer based on the ejb-multi-server quickstart is available here : https://github.com/wfink/jboss-eap-quickstarts/tree/test_overlay1
Please read the BUG_README.txt
Verified in 6.4.0.DR13.