The only solution I could really come up with was to not use the jbpm.sar and deploy the jbpm libraries directly in my ear. I even put the jbpm.war in my ear as well. I'm not clear on the reason/benefits of having jbpm deployed as a service with the sar file. If you search this forum, there was some discussion on the matter a while back, but I don't know if there was every any kind of "resolution".
Some additional info: Other sars/jars/ears/wars should *not* be able to see classes in eachother. So the solution posted by David is, afaik, the only 'solution', or you could deploy some required classes outside of your ear....