Thanks! I just took a look at AirTouch. It seems they have a nice ecosystem. Their Mitsubishi Electric offerings [1] use the CN105 port (like the OP's device).

I use the word zones on the Serin Multi Zone product page but it's interchangeable with indoor units. One indoor unit equals one zone.

You can override the room (intake) temperature per indoor unit/zone using a controller plugged into the CN105 port (like the OP wrote about). Depending on the controller, it can pair with a Bluetooth sensor or listen to any other type of sensor from Home Assistant. By going the ESP32 path, you can use simple, off the shelf $10 BLE sensors.

The Serin multi zone controller uses the centralized communication bus which isn't capable of overriding the indoor unit's room (intake) temperature. But if that is a must (which it is for ducted units), you can have both the multi-zone controller and a controller at every indoor unit without issue. So if you have an MHK2 thermostat on the wall and don't want to use their Kumo app anymore, the Serin multi zone controller will give you local control and you'll be able to retain the remote temperature sensor from the MHK2 thermostat.

[1] https://theairstore.com.au/product/airtouch-gateway-mitsubis...