In normal text (i.e not Markdown), using /slashes/ can get the idea across more visually. I think I picked it up on Usenet where it was reasonably common.
Most of the time, italics aren't actually all that important though, and could easily be replaced with other linguistic features.
Edit, another thing is that the slashes is actually graphemic notation, not phonetic. There's no reason that you have to use something modeled off the IPA there, it's just common to do so. You could use any alphabet.