Thats basically what hum does. The onstar device that has been in GM vehicles for the last decade or so is simply another device connected to the data bus and monitors for the airbag deploy message.

hum does the same, just plugs into the ODBII port which is a direct connect to the data bus as well and listens for the messages.

Onstar is bidirectional and can send messages on the bus such as unlock the doors or start the engine where I believe hum is read only only looks for accident related messages.