Well, we all know that tzaddi is tz/ts sound, and zayin is more of a st/z sound. The name Ezra is transliterated as ESDRAS and in Hebrew is ayin-zayin-resh-aleph. We also see the zayin in Ezekiel transliterated with a zeta, though... Hrm... I guess it is more complex than I thought. And what's even weirder is that Zephaniah is a sigma in Greek and a Tzaddi in Hebrew. Then again, this only shows for Hellenistic Greek, and none of this was translated until shortly before the end of the Greek era (sorry Bert, but there's only evidence for the Torah and possibly Ben Sira to have an early dating on them).
Ok, I recant what I said, go with Eureka's suggestion of looking at a Greek alphabet that didn't have zeta (if there is such a thing). And there's always the possiblity that it was different depending on placement, before certain vowels or consonants or constructions, sort of like how the Latin 'x' could be either gz or cs (lexi from legsi or dixi from dicsi).
Maybe we should commission a study?