I’m planning an ESS system and already have a Glow smart meter publishing import/export power to MQTT every few seconds.
Rather than fitting an ET112, can the Cerbo use this MQTT data as the ESS grid meter?
I know it probably needs to appear on D-Bus somehow, but has anyone done this already, or is there an existing project?
Or is fitting an ET112 still the only supported way?
Node red has a way to add virtual devices. One possible virtual device is an energy meter.
Using any virtual grid meters is not recommended. Communication Issues on any of the “complex” communication ways could make the meter values freeze, and make the inverters “run away” to their maximum charge / feedin power, while trying to “catch” that frozen 15W displayed, cause wifi was down, mqtt disconnected, etc…
A physical deticated connection is superior in terms of reliability and having a complete battery charge saved (emptied to grid, or fully charged at most expensive prices cause values were stuck) will already pay another meter 
Agreed there.
For some people it seems this matters less than saving a dollar or two and avoid buying a known working thing.
I would go as far as saying skip the ET and go straight to the VM.