The service does exist, but I have not used it.
Rosewood Mayakoba is the first hotel that will be offering VIP arrival service to all of their guests, up to four people for $150 total. Perhaps this will help lead the way to lower costs for the service in general, and open it up to more companies offering it.
I agree with others that you should be fine with an 8:45 AM arrival time, but if you are late it could be an issue. April 20 is school vacation week for NYC and BOS public schools. I am not sure about April 27.
Definitely use the Belmond transfer service at a minimum since you will need an early check-in.