The packaging plugin assumes that descriptors are named jboss-service.xml. In JBoss 4x (possibly prior versions as well), this assumption is relatively safe. However, in later versions of JBoss har deployments are no longer service archives and instead are supposed to be handled by the meta-deployment scheme. Alternate names are supported for the descriptor, but the plugin uses this configuration option to copy the named file to jboss-service.xml. This is incorrect because the deployment should not be treated as an MBean deployment.
I am suggesting a jbossVersion option for the har mojo that conditionally changes the plugin's behavior. The intent is behavior that is backward-compatible with the older scheme and simultaneously allows HARs for JBoss 5 (provided you supply a supported value for the jbossVersion parameter).