This link absolutely works for seat assignments, I used it for a flight last week and I just used it again today for a different ticket.

http://eu.flyasiana.com/I/en/Default...&menuType=LINK

However it is very picky with how you enter your name, it only works when I attach 'MR' to the end of my first name. If you have a middle name or middle initial on your ticket then you would also have to attach that to the end of your first name.