Don't even try to use Chrome or Safari to do Asiana seat assignments. It will pull up the reservation and allow you to select a seat but when it comes to confirm it with "Next" button it will just sit there - logging back in will show the seat was never assigned.

Mac users; use IE in Fusion or Parallels with a Windows VM

I've made seat assignment changes on OZ, UA, NH and SQ issued tickets and had no problem opening by ticket number.

Another thing you can do is setup an Asiana mileage account and open the record locator with the rec locator and mileage account. If the name on your mileage account matches the rec locator from Asiana it will let you do seat assignments. This will even work if the eticket info is not passed to Asiana properly (e.g. extra eticket digit). You must have the Asiana record locator NOT the locator from the issuing airline if it's not OZ.

You can still then alter your mileage preferences at check in at the airport