I think not. In 2006 we received a coupon from American Express offering a Saks giftcard if we would buy a Mercedes. The offer required that we use an American Express card to make a minimum downpayment ($500?), but explicitly stated that the dealer was allowed to limit the amount charged to the card.
AMEX will allow this. But not if Visa and MC doesn't.
Since Visa and MC don't then AMEX won't if that merchant accepts Visa and/or MC.