Reference (Open Source).pdf revision 1.13 (may not be latest) says for task::self(): "If the calling thread is a user-created thread that is not running any task, self() returns a reference to an implicit dummy task associated with the thread." So if you want to call task::allocate_additional_child_of() from your own thread, make sure it has an active task_scheduler_init registration, and use task::self().

(Added) It does leave me wondering what this does for ownership, because the owner may go away or become unregistered during the lifetime of the task. What does ownership do besides affecting the outcome of is_stolen_task()?