When your building out a new sim, sometimes you want to keep it a secret until opening day. But you can't hide from the map! Or can you?
I've taken a 256x256M megaprim, textured it with the "nothing here" color from the SLurl maps, and covered a sim with it at 375M. (400M seems to be the cutoff height for the map.) From this, I should know in less than two days if it "works". SLurls would still work, and I expect the pointer on the map would still show the sim's name. But you won't be able to see the build from the map.
I've never done any skywriting before in SL. From what I read, it seems the map averages the texture (and color) on a prim's face into a single blob, so you have to use individual prims to "write" things.