I was going to do this just to tow a car, and was told that each state was different in what they'd allow. The Washington state insurance people told me that I would have to get a Wa permit, then as soon as I crossed into Canada would have to find a insurance broker and get a Canadian permit? I'd question our ICBC rep as I always thought there was something about having 10 days to transfer a new vehicle. Maybe not the case anymore.
I was going to just buy a BC permit, which would be insurance and somewhat registration of the car in your name with the numbers on it. Being how many new US cars come across the border with a paper in the back window and no plates, I would expect all they want to know is that you have some insurance coverage, and a bill of sale with the car in your name. You might want to try an email to a sales person at a dealership in Seattle, and ask them about you buying a car there and wanting to drive it home to Canada. I'm sure they've all got experience with that! Good luck.