Toggling external midi via controller

I'm trying to figure out how to toggle on/off midi loops for external midi instruments via an external midi controller. Ultimately, I want to have all my loops running, and record the automation for enabling/disabling tracks to build an arrangement on the fly.

For softsynths I can successfully assign a controller button to mute via the controller assignments window. Got this running on a drum machine VST and can drop in/out kicks, snare, etc at the press of a button. This doesn't work for my hardware synths using external midi, as mute parameter doesn't stop midi being sent. For my external midi, I'm actually using a software instrument track and external instrument utility, as this give me access to midi plugins (arp, etc).

I've got the power on/off switch enabled for each track, but this doesn't seem to be accessible via a cc afaik. Toggling it isn't picked up by the controller assignments learn mode anyway. I can mute a region via the logic interface (control-M), but assume this can't be done via an external controller. Worst case I could assign my button to a volume CC on the hardware synth, but this is problematic dependent on which of my hardware synths it's going to, so would be better just to stop the midi note on/off getting sent in the first place.

Any suggestions? Thanks.