After chatting with Apple Support, the "Device ID" is clearly something that the card issuer generates. So, I contacted AMEX and eventually got to their "Digital Service" support center (800.297.7500). I got a pretty knowledgeable rep and explained that what I needed was the full Device ID number so I could register it with Rewards Network. After several escalations, in the end they said there was no way they were able/willing to release the number. They are not allowed to give it over the phone, were unwilling to use a Secure Message, and of course email would be insecure as well. They weren't willing to come completely clean that the Device ID was basically just another card number. They claimed over and over that even if it were a card number it would never work without the Apple Pay token used in conjunction with it. If I had the ability to see the number it would be interesting to do an online transaction (non Apple Pay) using the Device ID card number and see if it goes through.

In the end I guess it may provide some small sense of security that they are unwilling to give the Device ID out to anyone. However, earlier posts that it is visible using standard NFC terminal make me wonder if it is just a false sense of security.