Still hoping for some insight on this one. Andrew?
However, it seems that this is the way it was designed. My manager ran into the same limitation last year. I guess AC is not willing to pay NH for a business class seat on the route, but strangely they are willing to pay them to fly you from IAD to NRT in business on a codeshare.
Unless you're able to pay the premium to go via Europe, it makes it pretty crappy to go on AC from YYZ to BKK. You either have to spent 6.25 hours in HKG or do a milk run through Vancouver. Seeing the value of spending more time in the air than on the ground I'm going via Vancouver -- I guess it's not all bad.