I'm glad the OP got an exception here. 1500 USD to assign seats is crazy. I would have no issue if say BA were to block some of the J seats (maybe upto 1/2) for sapphire and emerald, leaving the rest of the cabin free to select for all. a last minute Gold in J on a status heavy route will still have a choice of seats from either the blocked section or the free for all section. If no choice available at that stage, then we could presume that the flight is full.