I'm sorry - Terminal C is US Airways only. Terminal B is for American and United. I'm not sure if the American (Terminal B) elite line is for UA elites too. The FlyerTalk AA Wiki page states:
"The middle pier [Terminal B which also houses UA at DCA] has an elite security line to the left side of the entrance maze for (and I quote): first class, business class, airline club members, Platinum and Gold members, oneworld and star alliance members. It is not always open." ~
http://www.flyerguide.com/wiki/index...rity_Lines_(AA)