That sounds rather risky to me. Just make sure you have a backup plan (ie Amtrak, bus, etc) ready, just in case there aren't any cars available. If you pick this at a peak time and/or when cars are going south for winter anyway, you might be out of luck.

There's typically threads on the car rental forums this time of year about companies giving deals on one-way rentals from locations in the northern US to those in the south. Fall/winter typically isn't peak season for Seattle tourism...