If you search for Mileage Plus award seats on ua.com there is decent availability AUS - YYC on May 20, 21, 22 ( those were the only dates I checked ). Mostly via DEN, but I did see at least one routing via SFO.

Any seat that is available for MP redemption should be available for AP redemption, shouldn't it ?

Find the flights you want on ua.com and then if they are not available from AP online you will have to use the call centre.