I did this by mistake at first and after a little playing around I managed to reproduce it.
Dunno if this works with a non epic flying mount, I would appreciate if someone tested that for me!
Steps:
1. Fly into the bank where indicated in the first screen shot.
2. Fly straight to the mailbox and don't stop until you hit the wall.
3. ????
4. Profit!
Image 1:
http://img86.imageshack.us/img86/23/...dinbankxr7.jpg
Result:
http://img515.imageshack.us/img515/4...inbank2ja4.jpg
Enjoy!