I currently have tickets booked through UA.com on AC metal. You can buy preferred seats if you go to your res through "My Bookings" and then click on the link for seat selection tool towards to top of the page (which will bring you to: https://services.aircanada.com/Servi...method=english).

I plan to purchase preferred seats soon and have gone all the way through the page where you would enter the CC info. Per my DMs with AC via Twitter yesterday, they confirmed that preferred seat payment for tix booked through other airlines should work. They had originally given the same party line mentioned by economyplus fan, but then when I told them that I could go in and select the seats, enter CC info, etc., I guess they checked and said that a "new tool" was recently added to allow it.