Ok, went back and read the whole thread. No where does it touch on why GW would be out of the picture. However as Crimsondude wrote the treaty of Denver covers alot more than Denver. It was the peace treaty that ended hostilities between NAN and the then USA. The treaty set the boarders between the countries, created the frame work for the migration of significant portions of the population and dictated that Seattle would stay an outpost of the then USA. The treaty has undergone two (?) revisions already. The first when the old USA split and the second when GW showed up and kicked the Auzzies out.
As various books have had shadowtalk about seperatist groups in Seattle for some time I would not be suprised if it did show up in Spy Games. I do think you would need s significant trigger such as a disaster in Seattle akin to what happened in New Orleans with Katrina both during the hurrican and afterwards. If the UCAS governement failed to aid the city again I could see it going much the same road as California in the Shadowrun world.