i wouldn't think that certifying the A319/321 in those conditions that much of a big deal . if the 320 can do it most certainly goes that the 321 can do it as well . (the roll out guidance system will move the rudder to try to keep the aircraft on center line as it tracks the localizer signal. if the 320's rudder can oppose the asymmetric reverse then it should be even easier for the 321 whose moment arm is longer , therefore more effective rudder )