What's been made available for the events from PubSub is nice, but currently there is no way to know of when a unfulfilled reward has been marked as complete by a Streamer/Mod. Since the event is already well structured, all that would need to be done is simply re-send the same event with the status property updated to reflect that it was fulfilled (either just using the existing "FULFILLED" value or possibly a new value like "MANUALLY FULFILLED").