Firewall/Security:
There seems to some focus on security in regards to the communication (dedicated memory locations) and we have implemented a "software" implementation (specific value within specific register before processing Modbus registers into internal memory location, eg. MHR2048=1234, run "Subscribe" events).
Within the Server setup add a list of "authorized IP and/or HW Address (MAC)" for publishing to the server address (MHR, MC, DL, etc.)
This is how a few devices from different manufacturers has addressed this basic security.
MQTT Broker:
I mixed terms... Meant broker, protocol converter.
Other than those, have developed a few custom applications and some "generic" deployments.
Keep up the good work.