Got myself a map here. The four big-league ballparks closest to PNC Park are in Cleveland, Washington, Baltimore, and Toronto--in that order too. The parks in Washington and Baltimore are nearly the equidistant from PNC. So are the parks in Cincinnati and Philadelphia.
Maybe the Orioles or the Blue Jays could be the Pirates' designated Interleague Rival. Uh, maybe.
The distance between Cleveland and Cincinnati is about the same (225 miles) as the distance between Pittsburgh and Toronto.
Also somewhat interesting: Seattle is easily the most isolated of the major-league cities. There's at least a half-dozen teams playing within 800 miles of Coors Field; Seattle only has two rivals (both in the Bay area) within that distance. Colorado's all alone out there, but at least they are in the middle of things. Seattle might as well be on a different continent.