I don't think such a thing could hold together very long, unless it was just a string in a line. Maybe in Morse code?
With multiple launches you could probably get several parallel strings, and use it like a dot matrix printer. It would be a heck of a stunt. But I wouldn't expect it to last for more than one orbit, and only part of the planet could see it.
You can change brightness and color of individual satellites as they move, so they would match the "pixel" they are in now. Just imagine swarm of very small emitting light bugs moving chaotically behind your screen and changing colors as they move from one pixel to an other. The only issue is to make sure that at every moment each pixel has enough bugs to get the required brightness.
With multiple launches you could probably get several parallel strings, and use it like a dot matrix printer. It would be a heck of a stunt. But I wouldn't expect it to last for more than one orbit, and only part of the planet could see it.