Hi all,
I have been looking for a solution for automatic accompaniment that would work on the Zynthian, (turning the Zynth into a sort of arranger keyboard).
It is also a topic that comes up regularly on this forum, so perhaps others are interested in it.
A solution on the Zynthian itself does not seem feasible—at least not one that is usable and of acceptable quality..
I came across a program that takes a different approach. It creates a backing track that you prepare in advance and play along with while you perform.
The advantage is that you can “compose” the timing and dynamics beforehand to create lively result.
It is different, but in practice, it works surprisingly well. The Windows program (open source) that I tried is JJazzLab. A good start is this introduction.
If you are interested in automatic accompaniment, you could take a look at this. It is worth it.
It is a very complete package that can read virtually any Yamaha style file (and there are hundreds of them). Broadly speaking, you can view it as a task-specific sequencer.
JJazzLab includes a built-in version of FluidSynth, and its own SF2 soundfont (it’s actually an extended General MIDI font).
You can create the accompaniment audio on your PC, or directly on the Zynthian by connecting the Zynth to your PC (via USB or DIN-MIDI).
I find it easiest –once everything is to my liking– to export the backing track as a MIDI file. You can then play this on the Zynthian. Each instrument is assigned to its own MIDI channel. This makes it very easy to use different instruments or apply standard Zynthian effects.
MIDI playback also makes it very simple to adjust the tempo. It’s a pity though, that Zynthian can’t play MIDI and record audio at the same time. I created the example below using a .wav export.
The program is very extensive (not expensive, it’s free), and it sometimes takes a lot of time to figure out how something works. If you have a Zynthian, you are used to that 
Note: JJazzLab is designed to generate backing tracks, not to host live instruments.
You can have your JazzLab play live chords via a split keyboard, but you must send the melody line to another sound source (a Zynthian, DAW, or something similar).
Besides a lot of hassle (and the need for a Windows device), this results in much lower quality, especially in the accompaniment.
A simple example (The harp is my clumsy contribution.):
Hope this helps someone.
Kind regards,
Hans.