This was a classloader issue:
I had the same class being loaded by 2 different classloaders - the web service's class loader and the MBeans classloader. The actual class was installed locally for both services. By extracting the appropriate class ( i.e. in this case 'com.NonSerializable') or interface into the JBoss lib directory ( i.e. $JBOSS_HOME/server/xxx/lib) this problem can be resolved.