Welp, over one year has passed and nobody's figured it out. Guess that means it's time to spill the beans. It relies on a pretty cool trick actually. Don't think anyone else discovered it yet.
First, activate the hand glider and head to this screen (http://www.mediafire.com/view/6e62wlkd6cbvh2d/Knytt%20void%20entrance.png). With a skilled jump, you can reach the spot seen on the screenshot and head below the screen. This'll bring you to the game's 8th layer of screens.
It's completely unused, safe for an impassable gray screen to the very left which is used for the death animation. Everything else is just void screens with a purple background. With a really difficult jump, it's possible to reach the 8th layer even without the hang glider, but then you'll fall to the bottom of it and die.
From here, you must glide all the way to the right end of the map. This'll take several minutes, but you can tell you're there when your character goes out of bounds. Now glide upwards to the sixth layer, then turn around. Don't do so too early or the temporary loss of momentum will make you fall too far. Note that all the darker water on the rightmost screen of the sixth layer is impassable, so don't be surprised if you end up clinging to it at the right edge of the screen. If that happens, just slide to the bottom of it. At any rate, glide upwards again. With a bit of practice, you'll bump against the darker water.