You can get F award seats on those routes during non-high season (using AA). Otherwise CX will make those available to CX Diamond only (at least this is the pitch that I have been given as to why I should become Diamond when I am already double emerald). Most likely some seats will stay unsold and become available less than a month before departure date; my guess is there will be plenty of short notice availability but very little otherwise this year. Given CX's need to maximize their premium revenue the rest of the year (and their ability to do so!) I personally think this is a fair and reasonable policy.