RJ and Finnair worked for me
 

Just updating the thread with my DP from today. I booked with BA Avios, but my relevant status is with Alaska. RJ would have worked EXCEPT they don't have AS listed as a Oneworld partner yet. With RJ I was able to remove my BA FF#, by selecting "None" as my FF partner. So, if AS had been an option, that would have worked I think. I tried Finnair without much hope as it has been hit or miss for me the last few tries (mostly miss). But, Finnair did work. I changed to my AS number and then went back to BA and selected my seats for no charge.

Just worked for me today. It took a minute and then when I went back into manage my booking, I was able to select seats for free.
One World Sapphire

Two questions for the board (booking a BA flight as AA Plat / OW Sapphire):

1) Does the free seat selection benefit apply to "Economy Basic" fares or only "Economy Standard" fares?
2) Does the free seat selection benefit include extra legroom seats, or just standard economy seats? If extra legroom seats are not included, is there a discount, or are they full price?

1) It will cover any Economy fare.

2) This depends, if flying Shorthaul in Europe the exit row is available. However particularly on A319/320 this may be lost should the J cabin reach these rows. Row 10 on 319 + 11+12 on the 320. For the 321 it depends on whether Neo or CEO. On the CEO the front exit row 10 can be in J, for the Neo row 16 will be in Y. Both rear exits regardless of type will always be in Y.

If flying longhaul the emergency exit is not available for free - will be full price - but the bulkheads are free.

Makes sense - thank you! Bummer about the exit row but so it goes.