Log in to your Marriott Rewards account and click Customer Support -> Contact Us via email. Then fill out the form. Although it looks like one of those generic forms that looks like it gets you nowhere, it actually works. I've passed on appreciations to a number of exceptionally helpful hotel staff and customer service agents this way before. In most cases, I actually received e-mails back from the hotel's manager or the call center's manager thanking me for it.