Wouldn't it just be something similar to the way you record your gameplay onto your PC with a capture card? The only difference would be that instead of saving it, it would just display it.
My DS upper screen is messed up, and I wanted to see what was going on without having to hold it in a certain position just to have a small red tint instead of the blinky white screen that I get if I stray the tiniest bit off from that position. I tried to find solutions to this, but all I could find was connecting it to a TV (and using a remote control instead of a DS to play, not what I want), and some stuff that looked like they were trying to convert their PC into a DS, and who knows what else. Why is such a simple seeming problem impossible to solve? (Well, at least it's impossible to solve it by looking it up on the internet...)
I can't remember how many times I've had to sign up for stuff to solve simple problems, and still getting no solution.