Flights continue as usual with planned weather diversions. The ash clouds are tracked by meteorologists just as are pacific typhoons or tropical storms which are circumnavigated.
On a northerly trans pacific flight, such as YVR-HKG, a preplanned, overall reroute further south from the Alaskan peninsula, or Aleutian chain, if necessary, would be insignificant. For 4-engined, non ETOPS airplanes, SFO or LAX-HKG routes are far south and wouldn't be affected by Alaska weather. Moreover, prevailing winds generally cause the ash clouds to drift east, and not south unto pacific routes.