i think you found the best solution that
1) does not change the jbpm db schema and
2) does not change the jbpm code.
more clean would be to write your own subclass of TaskNode, and add the members to that subclass and persist them with hibernate.
but that requires to change the hibernate mapping files (adding one) and hence changing the jbpm db schema.
ps. congrats ! you really know your way around jbpm.