I have tried to use 2 expired coupons in the last month; for the first Hyatt, they didn't bother to check the expiry date, and gave me the amenities. For the 2nd Grand Hyatt stay, they realised the cert expired; however I asked politely if there was any chance they could honor it, and they did. Nice box of chocolate, and a bottle of wine.
Try it at you next stay. Good Luck!