I can only confirm what most people have said in this thread; ask the cabin crew and they will go up to the flight deck and ask for you.
If you'd like to be regarded as a very rude character, take up DeltaMan's advice, by all means. In other words - don't. In my experience, cabin crew will go out of their way to make your flight as pleasant an experience as possible. If getting you to visit the cockpit will help reach that result, they will try their best to get you up front.
If you think trying to bypass anyone's authority is going to gain you any kind of sympathy however, I would say you're on the wrong track. Of course that's just my opinion. But a valid one, I dare say. I'm a cabin crew member myself, BTW.
I take it the A321 flights are medium haul in Europe. If so, don't wait too long to ask. Once service gets rolling, depending on how much time the cabin crew will have in flight, you'll be back on the ground before you know it. When you do ask before or during the service however (which is advisable even if it just were to let the crew know someone is interested in visiting the cockpit), just make sure to add "later on during the flight" to your request. Just a bit of practical advice there... Let them know you're not trying to stress them out.
Best regards and have a nice flight - or flights,