Two actually:
Easy(er): If your modbus device happens to be something like a PLC (or other programmable device), you could set up an ongoing "Handshake" between M3 and the PLC,
if the PLC sees the handshake timeout, then put a N/C block in your output rungs that open if the handshaking checks stop. You can use timers to set your timeouts on a "COM" drop.
Hard(er): From with in a Plugin, in the "MyNotify()", you can capture the "Shutdown" code "14", and then in that same block use the modbus data structures to send out,
a 1 or 0 across the bus to your device, that then in the device, that would shut its outputs down. In the "Init" part of your plugin, you could send out the oposite number across the modbus a 0 or 1 for instance, that would turn on your enable your outputs.
scott