I already thought about that but the problem ist that if the value reaches the threshold a second time within the throttled time you wont get alerted. Considering this my best guess would be that you query the "trigger_event.triggered_time" from the ".watcher-history-*" index in a chained input in your watcher and make your query-range of that watcher depending on that time-value.
(It would be an approach little bit similar to: https://discuss.elastic.co/t/watcher-alerting-time-issues-with-frequent-watch/68710/3?u=6tubp3gwd9zp)
Would that be even possible? If so how?