For future readers of this thread who do want to use MQTT 2, see MQTT 2.5 Event Bus for how to configure an event bus using the new binding. If you are using the Next Gen Rule Engine, I’ve submitted a reusable library to the Helper Libraries that you can just configure and use, no need to copy/paste/edit the code. The NGRE version has additional features like tracking LWT of the remote openHAB instances and the like.
I am looking to see if following the Homie convention would be useful for the event bus to enable automatic discovery. But I want to get the current version merged into the Helper Libraries first.