I tried to charge a BMW about two years ago. (I checked with AmEx first to make sure it would go through - could be embarrassing otherwise!) The dealer declined, nicely. I said, equally nicely, something about their agreement with AmEx to take the card as payment for ANY transaction. They offered my LoJack (already ordered) at no charge.

Worked for me. The LoJack would have cost me a bit more than the miles would have been worth, but cost them much less than they would have had to pay AmEx. (It was a small BMW.)

BTW, I don't think all Gold cards get 1.5 miles per $. Platinum cards don't. I think it's just a special version of the Gold card with a higher-than-normal annual fee. It may be worth it if you charge a lot on an AmEx card.