I think you can create your own WorkItemHandler for the Human Task node. The current implemenation gets the "actorId" and "groupId" variables, but you could write your implementation.
In fact this handler:
https://github.com/droolsjbpm/jbpm/blob/master/jbpm-human-task/jbpm-human-task-core/src/main/java/org/jbpm/process/workitem/wsht/AbstractHTWorkItemHandler.java
could have injected the peopleAssignmentHelper, to be able to use different strategies.
Hope this helps,
Demian