I couldn't find a way to change a plugin preset via MIDI learn in Ardour yet. So separate instances of Calf Equalizer will be required.
Secondly, if using only Ardour, I see no way of making a single instance of Zynaddsubfx output to different tracks/busses (i.e. different Calf Equalizers) based on the MIDI channel you play, since Zyn only has one stereo output.
If any consolation, note that Zyn has 4 system effects, so you could send each instrument (synth, organ, strings) to their own system effect inside Zyn, which could be equalizers.
I propose you create a separate instance of Zyn in separate tracks, each with an instance of Calf Equalizer and possibly other plugins. Just get a feeling of the speed and stability. In my opinion, this is not a bad option as the whole layout of the session is clear and understandable and easily tweakable and extendible even when under stage pressure
If you really want to only have a single Zyn instance with multiple instruments (on different MIDI channels), each outputting audio to a different Calf Equalizer, it is possible by using Konfyt.
https://github.com/noedigcode/konfyt However, the setup is slightly convoluted at the moment, but I'll describe it below.
In Konfyt, set up a patch per instrument (e.g. Organ, Strings, Synth...). Set up your MIDI keyboard to send program changes or CC messages to change patches.
In each Konfyt patch, insert a new MIDI out port, audio in port and point the audio port to a new bus.
In Ardour, you would have a track with a single Zyn instance containing organ, strings and synth parts, each on it's own MIDI channel.
Then, have a separate Ardour MIDI track for each part which sends MIDI to the main Zyn track, each on the appropriate channel.
Also have separate Ardour audio tracks for each part which contains the Calf Equalizer and other effects.
Thus, you would now have the following Ardour tracks:
Zynaddsubfx MIDI track
Organ MIDI track sending to zynaddsubfx track, channel 1
Organ audio track containing equalizer, rotary speaker, etc.
Strings MIDI track sending to zynaddsubfx track, channel 2
Strings audio track containing equalizer and possibly other effects (exciter, reverb?)
Synth MIDI track sending to zynaddsubfx track, channel 3
Synth audio track containing equalizer etc.
Now, finally, connect Konfyt up with the Ardour tracks as follows:
In each Konfyt patch, connect the MIDI output port to the respective instrument's MIDI track in Ardour.
Connect the audio in port to the main zynaddsubfx track output. (same for all patches).
The audio in port sends to its own bus. Connect this bus to the respective instrument's audio track in Ardour.
Finally, you would have this signal flow for each patch.
MIDI keyboard --> Konfyt MIDI in port --> Ardour MIDI track --> Ardour Zyn track --> Konfyt audio in port --> Konfyt bus --> Ardour audio track
