Seattle's waiting room is substantially smaller than most other hub stations where they make you line up. IIRC, there's only one door out from the waiting room to the tracks (there might be one other, but it's very close and easily seen by the Amtrak employees guarding the line).

If you could get yourself out to the platform without waiting in line, there wouldn't be any negative consequences to skipping the line, but I'm not sure if it would be possible to do that at SEA. I don't even think you can walk around the building, as the tracks enter a trench/tunnel immediately to the north of the building, and, as I recall, the platform is fenced in to the south of the station (see http://g.co/maps/hzhnn).