So, my first thought is that one of the primary purposes of OH is to bridge between different technologies. So why not just bridge Zwave to MQTT using openHAB? With the event bus configuration (see MQTT 2.5 Event Bus) all it takes is a few lines of code (or download from the Helper Library if using Python) and a couple of Groups and all your zwave devices will be happily pub/subing to MQTT. And then you get to take advantage of the Zwave support in the openHAB binding, which, based on what I’ve seen written, is second to none in terms of number of devices supported and ease of use.
Personally, I think moving to something external to OH just to “MQTT All the Things!” seems like cutting off your nose to spite your face. It’s going to be a lot more work, you will have poorer support for Zwave over all, and you won’t have really gained anything except yet another little service you have to monitor and maintain.