So you have the right idea, unfortunately this vehicle is a bit unique. This is because the Body Control Module is responsable for unlocking the doors AND receiving the RKE key fob commands so there is no reason for there to be a CAN BUS message to control the locks on this using the RKE messages. So don't bother with pressing the key fob and trying to find a command to replicate, this won't work.
HOWEVER, OnStar has a message it sends to control the locks. So you can simply watch for that message:
Unlock Drivers Door is: 0x1024E097 00 02 FF
Unlock All Doors is: 0x1024E097 00 03 FF