As a data point here is my facebook message to AC last month about it:

Apr 17th, 8:14pm Hi There, if I am purchasing an Air Canada flight (AC792 on May 4th) through United Airlines am I able to call in or message and purchase the preferred seating? or will it only be available at check in? I'd like to purchase 12a and 13a on that flight



Hello, Unfortunately the Preferred seats will only be available to you at the check-in. Regards, Air Canada Social Media /aa