Lock Screen interface with some programs

The Ubuntu lock screen currently lacks a method to interface with certain running applications in limited ways(Change volume, etc), which could be useful according to the activity at the linked Ubuntu Brainstorm post. As mentioned there, this is even more useful on touch devices that lack a keyboard, making it difficult and cumbersome to log in to simply mute the speakers. Applications should register "widgets" to run on the Lock-screen and interface with the application(whether via Dbus, IPC, cans and string, etc) which will be used to make limited application changes like changing volume, possibly dismounting removable media, etc...