The 321 has no outer tank, so I don't think it is a big deal to go with less than 750 Kg in the outers in a 320 as long as you have enough fuel for the flight.
I've read/been told that the A321 wings are structurally different from the other A320 family members and physically come from a separate manufacturing process (Can anyone confirm?). So would assume that limits from the A320 should not be applied to the A321 carte-blanc.