In my experience, when you cancel CX space using BA miles, the seat does appear immediately in the inventory that AS has access to, so this strategy should work. However, if one has the BA miles to book CX in the first place, keep in mind the massive BA devaluation for partner bookings scheduled for April 28. Might be prudent to just burn the BA miles. BA Fuel surcharges are usually less than $100 traveling to HKG, 3 x that much on the return.