I also use node-red with a serial gateway attached. I created a flow that converts the serial payload to a mqtt type payload and it is picked up by openHAB. This also works the other way around. openHAB sents everything out via mqtt and node-red picks it up and parses it back into a serial formated payload which is picked up by the serial gateway. I also use node-red's Owntracks node to monitor my proximity and send updates to openHAB as an mqtt message and I also have a flow that monitors my phone and my wifes phone, if we are both home the outside lights get turned off at 22:00. node-red also sends twitter messages to me when my doors and windows are opened and no one is home.
I only use mqtt with with a combiniation if nfr radios via the serial gateway, like I mentioned above, and I also use ESP8266 modules running lua. So as not to send the wrong payload to the serial gateway, all topics that come from openHAB that are destined for the serial gateway start with RF24SN/out/... and on node-red I subscribe the RF24SN/out/#. This will capture all those message and they will get parsed to the serial format (Nodeid;clientid;message;...etc).
Everything that happens in node-red is passed to openHAB. I have a couple of relays and leds I control from openHAB. The leds use the MySensor LEDDimmer. Basically node-red acts as the middle man between the serial gateway and openHAB/mqtt.