You could probably add that if you were sufficiently motivated. Either by adding an LED on the power or data path, or by adding a physical switch. I think it should be fairly easy on laptops; I'm not sure where you'd jam the hardware in a phone or if you can access the cables for the camera/mic without messing with traces.

I'm a little curious if iPhones could be modified to route the {mic,camera} {power,data} through the silent mode switch, either instead of or in addition to the current functionality. I don't really have a need for a physical silent mode toggle, I'm totally fine with doing that in the settings or control panel.