Hi Thorsten,
as far as I know nothing happened in this direction yet, but MQTT is a topic that comes up very frequently. I am personally lacking in-depth knowledge about MQTT so I am not able to design a good software architecture for OpenEMS yet - but implementing it as a “MQTT Bridge” just like we have for Modbus/MBus/OneWire/… seems to be the right way to go.
As MQTT is an important topic with many possible use-cases I would like to suggest that we work on a proposal to clearly define it (just like we did for Edge-to-Edge communication) @p.wimmer, @c.lehne and @zoernert might be the good people to talk to about this.
That MQTT Broker for GSI you are talking about: Is it a software product or a hardware device? Maybe even integrated in Discovergy smart meters? Can we try it?