For example, tevent does not support timed io events like libevent. Not sure about libev or other event backends.

ns_job_t already has pointers to multiple event fw events:

    ns_event_fw_fd_t *ns_event_fw_fd; /* event framework fd event object */
    ns_event_fw_time_t *ns_event_fw_time; /* event framework timer event object */
    ns_event_fw_sig_t *ns_event_fw_sig; /* event framework signal event object */

ns_add_io_timeout_job could allocate ns_event_fw_fd and ns_event_fw_time, then the event_cb could determine which one of these should be used, and set the job output_type for the actual job callback.

Or, we could push this functionality into ns_event_fw.h - have new ns_event_fw_t functions for ns_event_fw_add_io_timer, ns_event_fw_modify_io_timer, and ns_event_fw_io_timer_event_done