I think a bit of talking in circles here, a Mach Trim system is usually a 'Stand Alone' system, typically achieving it's goal through Elevator movement.
woderick's input contained the vital clue as to how later generation aircraft compensate for Mach Tuck - The Mach trim mode of the stabilizer trim system provides incremental horizontal stabilizer position in response to changes of Mach.
Not a Mach Trim system per se, but a MODE provided by incremental stabiliser movement as directed by the AFCS.
I cannot speak for the B757 (sadly), but the Mach Trim MODE (not system) is very much in evidence on the B767 and B777.
Regards,
Old Smokey