I am honestly amazed this question has been brought up so recently. While upgrading my home security system, I had the same thought about my phone. My house has a silent alarm feature and other measures for duress, why not have the same for my digital house. My linux workstations have the same feature triggered by which user logged in and start-up scripts. It would be interesting to have the same feature on my phone yet I understand how difficult testing would be since this action is irrevocable. If I had access to a virtual version (with snapshots) of my phone I actually would not mind building, compiling and testing out a feature like this.