A solution might be to add your jar file directly into to your ear.
Then each module (EJB,WEB,...) using this library must have its classpath modified to reference this library. This can be done in the MANIFEST file of each module.
Example of configuration:
bertou$ jar xvf MyApp.ear
bertou$ jar xvf lib/MyEJB.jar
bertou$ cat META-INF/MANIFEST.MF
Ant-Version: Apache Ant 1.6.2
Created-By: 1.4.2_04-b04 (Sun Microsystems Inc.)