Just another data point: today I made a booking on finnair.com and entered my BA (Gold -- OW Emerald) FF number. After confirming I went back to 'My Booking' to select seats. The tool wanted to charge me. I rang up the Finnair Plus Gold helpline and the very friendly, very helpful chap there assigned me the exit row / extra legspace seats I had chosen, at no charge.
cheers,
Henry