I don't know all of the restrictions, but you could purchase 2 e-fares for this weekend. Most of the fares are departing on Saturday, return Monday or Tuesday.
GSO-PHL 189 r/t USAir
PHL-SEA 289 r/t USAir
Then take the train option or I imageine weekend airfare between SEA-PDX can't be more than $150. Travelocity pulled up $217.
Another option, CLT-DEN is only $158 r/t. It is not PDX but is closer to your destination and Frontier may have driven prices down there.
To search for fares and more info:
www.webflyer.com/deals/