By setting a client's see_invisible to a text value, things are displayed just as they would if it was a number. Objects that are not invisible are seen, objects that are invisible are only seen if your see_invisible is equal to their invisibility. The only difference between having textual invisibility as opposed to numerical invisibility is the rule that states having a see_invisible greater than or equal to the object's invisibility displays it.

I am requesting this because I want to have certain static objects that remain on the map for clients to see moving, etc without having to create images.