In which case they usually isn't a problem. Only BAEC status holders can select seats before-payment-without-seat-payment. Everyone else needs to enter their oneworld card during the initial booking and then at the end, they can normally go to Manage My Booking to get the right seat. Obviously it's best not to pay for seating along the way. In the unlikely event that your status isn't picked up in MMB then either try the hacks suggested up thread or give BA a call. I'm only aware of one issue here, namely when there has been a recent change to AA status and BA hasn't picked it up yet, but if you have had Sapphire or Emerald status for more than 48 hours there shouldn't be a problem.