I am using JBoss 4.0.2 and I am able to deploy the Hibernate HAR files properly. The only issue which is seen in this version of JBoss is , the HAR can not be used as exploded directory .It does not work simply.
So work around is to use the archive instead.
If you are facing problems with even the archive format, then you need to check your hibernate service.xml and also the JAR versions.
One more thing to note is you will have to add commons-lang.jar for JBoss4.0.2 + Hibernate applications. .Surprisingly , JBoss does not ship with this jar.
You can get this jar with one of the hibernate sample applications.
I hope this helps.