I don't think you'd have to project black on the screen, I think you'd have to write an app that worked like a screen saver on a laptop. Whereby, after a specific time (2 minutes, etc...) the phone "shuts down" and goes into screen saver mode, at which point the only thing running besides background apps or whatever else runs would just be the little 'whatever' moving around the screen. It would have to work like a screen saver on a laptop or something where it's shut down to the point where it's saving power but the screen is still technically operational. Basically, it would be doing whatever it does when it locks the screen down but without actually locking the screen down. (I think the only reason for the screen lock is to prevent doing something on your phone via an accidental touch. Is this right? If so, a small moving object on the screen should almost completely prevent accidental phone usage.) A small something moving around on the screen shoudn't be much of a power draw at all. Even then, you could set a longer time (2 hours, etc...) at which point even the "screen saver" would stop and the phone would lock down.
I would think you would have to just write it as a screen saver app, and then in order for it to work right, you'd have to change your phones setting to setting>sounds and display>screen timeout> never. The app would have to control the setting like when to go to the screen saver and when to lock down all together.
Is it possible???