I am not familiar with the interface on these devices. MIDI clock is continually variable and will clock at 24 pulses per quarter note so the rate of clock changes with tempo set in Zynthian. If there is some other workflow step within these devices that is easier with its click detection than its MIDI clock slave then I feel they have missed a trick and you will have to create a click track, e.g. by adding an layer with a simple synth and creating a pattern in zynseq.
I did some benchmarking which showed Calf monosynth to have quite low resource use so that could be a candidate but… what audio output do you plan to use? Surely you want to use the Zynthian audio outputs for Zynthian synthesis / audio processing?