I was at school with a girl called Zephne - pronounced like Daphne but with a Ze instead of a Da. But where she got it from I don't know. If anyone's got a clue, I'd be interested.
As foryour question, it might be useful to think of possible German names - the single S in German is pronounced like the English Z (and the German Z like a TS), so people who didn't know German and heard a Z sound might quite innocently give it an English Z spelling when they wrote it down and confuse the rest of us decades later. Something like Josephine, perhaps?