Welcome to FlyerTalk, Vancouverite. It may be possible under existing Priceline/Northwest rules, but I don't believe it is. Priceline tickets are extremely restrictive, but miracles do occur. For example, make sure your WorldPerks number is in your reservation, just in case.

I would be very pleasant and ask at the ticket counter, WorldPerks Club and/or at the gate if I may fly standby on an earlier flight without any additional charges. I would not mention that I am flying on a Priceline ticket. They have that information available to them.

Good luck and happy travels!