Funny I just experienced the same issue with seat selection for an AS award ticket. No way to add my personal contact info and can't add seat. I'd love to know what you find out from their customer service.

For what's worth I also have a paid ticket directly booked on their website, and there is no way for me to add a seat. The is a button on the website but when I click on it a popup window says "Action not allowed We are unable to make changes to your seat selection for this flight. For assistance please contact our Call Centre on (+968 2453 1111)". I remember there was an option to pay for a seat during the booking process, but decided to wait hoping once they became part of OneWorld I'd be able to get a choice seat for free. Unfortunately that didn't pan out as they have moved out the join date to June next year...