Just to clarify for others reading this post in the future:
The problem described in this post is between Guvnor 5.4 and JBoss AS 5. (jBPM happens to be involved but is not related to this problem).
The answer might be described here:
https://community.jboss.org/wiki/DeployDrools-Guvnor540CR1ToJboss-eap-512