ok, now I got it, thanks for explanation :)
one way of doing it could be to utilize additional participants in the task life cycle defined by WS-HT spec, for instance task stake holders. And then you can query for tasks that are assigned for task stakeholders using method: taskClient.getTasksAssignedAsTaskStakeholder
unfortunately task stake holders are not filled in by out of the box HT work item handlers in 5.4, it was added after the release so you need to extend them to accommodate such feature.
HTH