The package org.jbpm.bpel.integration.server and all classes in jbpm-bpel.jar are independent of the target app server. To port jBPM BPEL to a new app server all you need to do is implement the EndpointMetadataLookup interface, register the new class in endpoint.metadata.lookups.xml and rebuild jbpm-bpel.ear.
To provide web services, jBPM BPEL requires access to the WSDL document and other metadata not available through the JAX-RPC API. This is why the metadata must be retrieved in an implementation-specific way. The ubication of this metadata changes even across JBoss AS versions. In particular, it changed radically from 4.0.3.SP1 to 4.0.4. You see, the portability issue is neither unexplored nor hard to tackle.