This is the case with almost all the non-US FF programs so I expect A3 will be the same, not allowing O class (F on US/UA) as part of an I class (business) redemption. So either we will need to accept economy for the domestic portion or we will need to pay more miles or we will need to find I class options. CO and AC use I class on all N America routes and UA does on 3 class routes like PS. UA has also been known to load I class in addition to O class on some 2 class planes on routes with a lot of international connections such as SEA-ORD; if it shows available in I class (business class award inventory) then I would expect A3 can book if for you as part of a business class international redemption.