While I was in the middle of writing a solution to handle multiple Modbus TCP slaves over MQTT, I updated my RUT900 with the latest firmware (RUT9XX_R_00.06.05.1) from 2019.11.15, as I was intrigued by the new "Modbus TCP custom register block" support in the Modbus Slave. Only to find something more interesting a new tab on the Modbus settings page: MQTT gateway.
Activating this enables the process /usr/sbin/modbusgateway with the default arguments 127.0.0.1 1883 request response.
Now I was unable to find any documentation about this feature. How it is intended to work, how to interact with it etc. I assume it is intended as a direct MQTT <-> Modbus gateway, where requests over MQTT results in Modbus TCP (RTU?) requests which in turn results in responses over Modbus back to MQTT. If this is the case then great, while my solution is already done, it would be better to use a solution supported by the base firmware.
Does anyone have any information about this feature?