Any chance to make it possible to build special WinRT application that would run in lock screen mode on Windows 8 like it "works" for default Slide show option setting?
You can do things with Badges, such as outlined here and here. Unfortunately, I don't think you can do exactly what you are asking, aside from possibly disabling the Lock Screen and building a facsimile of your own, though I don't know to what extent this will be allowed in the store, and will likely be largely up to the users settings with regards to things like working on battery or not.