For AF/KL redemptions you should be able to select seats. With no status, you might need to pay for them. For other airlines, the policies of the operating airline will apply and you might need to call them or use their website to select seats. The PNR for the two airlines might be different, and you can get the PNR for the operating airline by calling Flying Blue.

The website is relatively glitch and sometimes will say no seat selection even though you can select seats. If you're worried, call in to Flying Blue and see if they can confirm seats will be selectable.