The last time I went through Seattle security (end of this past November), the lines wrapped through that large hallway area. They were sending some people through the C gates and others through the N-gate security regardless of who or where you were flying.

I doubt you would get turned away, but as another mentioned here, the rules do change... and often. So don't be afraid to ask a friend Washington security guard!

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"What do you mean you didn't get miles for that?!"