If you can't find the list, simply put any large part 121 operation likely has reciprocal agreements with every other large part 121 operation.

I'm guessing but, PCL pilots can probably jumpseat on United, USAirways, Northwest, Delta, Continental, JetBlue, Air Tran, Southwest, Alaska, America West, and all of their various express carriers plus some smaller regional airlines, cargo, and even possibly part 135 operators.