Hi Hans,
I got no feedback on this issue ever. Finally I had to patch the jbpm code to allow subclassing of the TaskService, see
https://issues.jboss.org/browse/JBPM-3074
and implemented custom SeamTaskService, which provides a method for replication of user/groups database from Seam IdentityStore.
I have no idea whether JBPM-3074 patch is included into jbpm though. We do not use jbpm any more. I have tried using multiple open source BPMN engines and I found that the common approach, the "executable XML" is not practical, at least for me. I prefer good old plain Java instead of complicated puzzle made of plugins, property dialog boxes and code snippets written in scripting languages. I implement BPMN nodes as annotated POJOs being handled by a simple BPMN engine, which consults BPMN XML only to route tokens between the nodes (POJOs) automatically instantinating and persisting the nodes.