Called Amex regarding their list of participating properties. Agent said she couldn't send me a better list. She referred me to the Marriott website which leads me to believe that there are no non-participating properties. She said she would give my feedback to her manager regarding the list.

Anyhow, she did give me a tip on how to search the list. She said to press Ctrl+F and a search box comes up and you can enter the name of the property or city you are looking for.

Sorry, that's the best I could do this time.