We have something similar. With a twist. Once the task is created and the timer is set, it must be possible to adjust the date for the timer to fire. E.g. we have a timer that creates a task for a supervisor to investigate once the original task has been in progress for a week. But if the original assignee gets sick and can't work, we need to adjust that timer.
I had no alternative but to set the duedate of the timer directly in the jbpm4_job table. It works well, but it would have been a lot neater if the API provided some way of manipulating jobs.