You could kind of do this with a phone. Hit an emergency button and it starts discretely recording audio remotely. It wouldn't be overt, but you'd get a lot of evidence.
What it would not deter, though, is a worst case Martin scenario: he sees that Zimmerman is wearing the device and without words ambushes him from behind, making sure it never gets to transmit a clear picture or useful audio of him.
Which strongly argues for covert monitoring, the same reason a lot of people prefer concealed to open carry. At net such an overtly visible system might get more people hurt and killed....
Of course my blog post is not taking into account every single possible horrible scenario that could have happened, how could anyone debate all the possibilities? I merely state the case that this new technology should be explored for ways to prevent escalating tensions.