Phil Boswell:
Something like a tilde surrounded by double square brackets would probably be appropriate:
[[~]]
Please, no cryptic symbols like that. The point of a toolbar is to make it intuitive to format wikitext. A newbie neither knows what double square brackets mean nor what "~" could stand for. The blue, underlined "Ab" works perfectly well. Sure, it's Latin characters, but they are internationally recognizable.
Best,
Erik