A biblical Unitarian response found here says in part
Translators and commentators who believe that the word “pierced” should refer back to the pronoun “him” cite textual variants that more clearly read “him,” as well as the flow of the sentence which continues with the word “him” in the phrase “they shall mourn for him” and “grieve bitterly for him.” The Jewish understanding of this verse has always been that the one pierced was one in an intimate relationship with God, but there is no record of any early Jewish commentator understanding Zechariah 12:10 to be saying that somehow Yahweh Himself would come into the flesh and be pierced in the literal sense of the word. It is apparent to us that the Revised Standard Version has a good translation of the verse and that Zechariah 12:10 is a prophecy of the piercing of the promised Messiah. Another important point to make is that Zechariah 12:10 is quoted in John 19:37 after the Roman soldier thrust his spear into Christ’s side. John 19:37 reads: “and, as another scripture says, ‘They will look on the one they have pierced.” The King James Version translates John 19:37 as follows: “And again another scripture saith, ‘They shall look on him whom they pierced.’” The different versions may disagree on the Hebrew text of Zechariah 12:10, but none of them disagree on the translation of the way it is quoted in the New Testament. None of the versions have the word “me,” and most of them supply the word “him” as does the KJV, NASB and RSV. If the original reading of Zechariah 12:10 was “me, whom they have pierced,” we can think of no reason that it would not be quoted that way in the New Testament. On the other hand, if the reading of Zechariah 12:10 in the RSV and other versions is correct, then it makes perfect sense that the verse would be quoted in the New Testament the way it is. We contend that the New Testament quotation of Zechariah 12:10 gives us the proper interpretation of the verse.