Cathay Pacific do not allow advance seat selection on economy class redemptions made using Asia Miles if you do not have oneworld status (BA Bronze/Silver/Gold; CX Silver/Gold/Diamond), so I'd assume this is a blanket rule regardless of which airline's frequent flyer program you made the booking with.
I recently booked an award flight using Avios on CX - I earnt BA bronze status between booking and travel and it enabled me to select a seat.