Hey Jamie,
Wow - good for you & such a great idea! I double checked and yes they are accepted at locations in Mexico. Some things that I noticed in the terms and conditions of the cheques are:
- Marriott Cheques may not be acceptable at concessions within the hotel not owned or operated by Marriott or Renaissance. Please confirm with the hotel prior to making the purchase.
- If a registered guest, charge goods and services to the room and present the cheque at the front desk at check-in or check-out. If a non-registered guest, present the cheque at the time of purchase. The total charges will be reduced by the face value of the cheque. As they don't give you change from the check it is best for you or you guest to use the whole value of the check at once or charge the drink to your room and then you can use the checks upon check out.
I really hope this helps if it not please let me know as I am more than happy to help with any questions you may have.