I had a balance of about -15,000 on the card a few weeks ago. Again this was unplanned, and I had slowly been working my way down from about the -30,000 it had been on. Then I got the card fee increase letter, and cancelled my card.
There was no problem with the negative mileage, and so far I have not heard anything about it. I have the miles in my BA account, and would be happy to sign them back the Amex if required, but I get the feeling I might get away with it.