Good question - these are car rental charges booked and confirmed in €, with a closing invoice at drop off shown in € (although the card is not scanned again) but being billed in $.

Just an an update to the response from AmEx customer service - they have explicitly said that DCC is supported and that the customer should be presented with a choice on the terminal at the point of sale. Really surprised by this response and it implies a big change (I think) to the merchant agreement and undermines one of the reasons I use my AmEx over other cards when travelling.