Thanks, Maciek. I don't have much free time to test it thoroughly at the moment, but a quick test resulted in class not found exception (org/jbpm/integration/spi/DeploymentAdaptor). I have placed jbpm-jboss4.jar into the lib and added JBPMDeployer into jboss-service.xml:
<mbean code="org.jbpm.integration.jboss4.JBPMDeployer"
name="org.jbpm:service=JBPMDeployer">
</mbean>
I'm not sure if exception is related as there is a strong possibility I have accidentally broken my setup.
I'll get back to you as soon as I'll get some time to play with jbpm.