To the best of my knowledge, a normal AudioUnit (i.e. not an AUv3) can not have a MIDI output that is visible in Logic. This is a function of Logic, not AudioUnits.
Thanks crandall1, as it turns out we did find a way to register the MIDI output in Mainstage.
In CoreMidiHelper::setUniqueIDForMidiPort() (in juce_mac_CoreMidi.cpp), we changed the line
if you want your standalone application to act as an “external MIDI” device for Logic or Mainstage to send midi messages to, you need to use MidiInput::createNewDevice()
This question was about an AUv2 plugin, not a standalone (sorry, never clarified that).
Essentially, the AUv2 creates a MIDIOutputDevice that is visible in Reaper but not MainStage / Logic. To make it visible in MainStage, we have had to rewrite one line of CoreMidiHelper (see above).