John, your guess appears to be correct, but unfortunately I'm only a little closer to solving this mystery.
From what I can tell, the notation should be either {0:Label0,1:Label1...} or {0:Label0},{1:Label1...}. But neither work.
If I write {0:Not at All}, I get the desired result for the first option, which happens to be 0. But I can't get it to work beyond the first option having tried a dozen variations. In fact, if I write {1:Not at All}, there's no label and all the tick mark numbers disappear.
My guess is either there's a bug or the slider tooltip is only meant for formatting the numbers and not masking them with labels.
