There is an interesting tidbit of information that does make for interesting reading:
"In addition to detecting events on the aircraft and sending messages automatically to the ground, initial systems were expanded to support new interfaces with other on-board avionics. During the late 1980s and early 1990s, a datalink interface between the ACARS MUs and Flight Management Systems (FMS) was introduced. This interface enabled flight plans and weather information to be sent from the ground to the ACARS MU, which would then be forwarded to the FMS. This feature gave the airline the capability to update FMSs while in flight, and allowed the flight crew to evaluate new weather conditions, or alternative flight plans."
ARINC 619 details the protocols that are used to send data to other avionics packs on the plane, however that does not necessarily mean that a message could be sent to EXECUTE a command on the FMC.
What this means is the assertions that ACARS does not in any way interface with other avionics on board (which is what I always believed to be true) is not the case.
Where there is a datalink or common bus between two systems, there is a vector...
Food for thought.