As documented (http://docs.jboss.com/jbpm/v3/userguide/taskmanagement.html#theidentitycomponent) you should remove some mapping info from the hibernate mapping file.
In addition, the best way at this moment is to provide your own AssignmentHandler implementation. You can then feed this handler a configuration string or a number of configuration parameters that you can use to do the assignment. Contrary to what is mentioned in the docs I think it is not yet possible to change the ExpressionAssignmentHandler by your own implementation. AFAICS this is not yet configurable. Tom (or someone else) correct me if I am wrong here.
Can you give me an idea of how to do that?