Tommer, for what you describe human task notifications sound like perfect fit. They are part of deadlines configuration of a task. To be more precise start deadline. In general, start deadline is reached when task is not started before the given time expires. There are two options for deadline escalation:
Both can be used at the same time, but in your case notification is more interesting.
Notification (email notification) can be sent to some recipients that are not assigned directly to a task, which could be what you are looking for since there is no user registered at the moment.
Unfortunately there is no out of the box support to define deadlines in BPMN2, at least I do not know it. So extension of *HumanTaskHandler class will be required to setup deadlines on the task. Some additional configuration is needed on Task Server side as well, for details about task server please have a look at: https://github.com/droolsjbpm/jbpm/blob/master/jbpm-human-task/src/test/java/org/jbpm/task/service/TaskServiceDeadlinesBaseTest.java
HTH