Sorry for the late followup update, but here it is.
The problem with the Dice MIDI port made available through the firewire driver is that it disabled by default. When switching on the Harmonizer the audio over firewire works, but the Dice MIDI port is dead. On OSX, in the Audio MIDI Setup app, the Event Dice icon in the MIDI window shows grayed out.
Now a 'Rescan MIDI' command in that same MIDI window fixes the problem temporarily, the icon lights up and the Dice MIDI port becomes available in my DAW software. And it works too.
However, the moment I power cycle the Harmonizer the MIDI port is disabled again, and everything repeats.