Oceanic Clearance
Unlike the Atlantic, there is no "Oceanic clearance" requirement in the North Pacific. Read the NOTAMs ref NOPAC routes, file a flight plan and call Seattle ground for a departure clearance. If you can fly at FL410 or above or FL280 or below you can probably ignore the published tracks and fly whenever you want and even try for random or direct routes. The best advice is to call the Oakland Center (ARTCC in California) Traffic Management Unit since they publish daily tracks based on winds etc. and they have a basic idea of the number of aircraft flying and competing for requested altitudes in any given time slot. Sorry I don't have their phone number but look on line at the FAA website or in the phone book. Hope this helps and if you're flying through Anchorage Center airspace in the next few days maybe we'll talk on the radio.