Uh... did you read the manuals? It's in the Microbrute Connection Manual, in the Changing parameters via MIDI CCs chapter: Local ON/OFF is MIDI CC 122. It's not clear which value sets it to local off, but it does say that it takes the values 0 and 127 so it's just a matter testing which one does what.
Ha! I missed that. I assumed that the midi chart would only represent the stuff that's in the UI. Now you mention it, yes local on/off sounds familiar from other stuff, it's been a while since I used any outboard gear in anger though :-)
My bad and good spot Bacon.
Still, odd that it's not in the UI.
Also, as the only way to set it is via midi in a DAW, it probably won't get saved when you turn off the MB as the manual states that you have to use the editor to save any changes, just using midi from a DAW means settings won't get saved.