Wide open availability through the end of May (inclusive of Memorial Day w/e). 13,300 miles through NRT, though there are some other routings available. With the current 2x RDM promotion, this drops to around 2.3/cpm.
Also, the double miles promo is not exclusively for flights from PDX. Any US-CAN seems eligible (requires preregistration), so that one could start from any DL city that offers an attractive fare.
Please remember than China requires a visa in advance for US and most nationalities, except for some transit passengers.