Hey Baggypants,
it’s normal that you can’t see it on your iPad. Both the iPad and the Zynthian (with rtpmidid) are session listeners. You need a session initiator in front of the daemon to invite it and negotiate the session.
If you have a Mac, you can use the Network MIDI applet in MIDI/Audio configuration (in System Tools) to create such a session
If you have a PC, you can install rtpMIDI driver from Tobias Erichsen. It has exactly the same user interface than the Mac driver by the way.
For details about the configuration, I recommend you to take a look here, this is the document I wrote for KissBox a few years ago to explain the use of RTP-MIDI https://kiss-box.nl/downloads-archive/integration-guide/
Note that rtpmidid daemon is able to become a session initiator (my RTP-MIDI source code contains everything for that), but it would need to be modified to accept arguments (becoming a session initiator requires to know the IP address and the UDP port to invite on the remote device). I did not implement this yet, because I am not sure it would be easy to enter an IP address and a port number on the Zynthian GUI.
If you think that this would be feasible, then I can implement command line parameters to handle this mode. Give me your feeling about this.
Benoit