So, from what I can see, the connection is actually made by the Apple Audio MIDI setup rather than the other way around.
So I need to advertise the _apple-midi._udp service.
Step up pybonjour ( http://code.google.com/p/pybonjour/ ). Note that I missed this handy hint about a dependency, when I was headscratching about why it wouldn’t compile and install: “Under Ubuntu, the package to install is libavahi-compat-libdnssd1.”
I can run the register_service.py example script and see my service appear in the Audio MIDI setup.
Every journey begins with a single step !