Rabbi Moshe Miller of "AskMoses.com" responded to my inquiry on Tashlich with the following thoughts. First there is no mention "at all" of tashlich in the Talmud. Secondly, tashlich is found in the Zohar and the reference is Volume 3, page 101a. Third, Rabbi Miller said that some rabbis see "an allusion" to tashlich in the book of Nehemiah (Chapter 8:1-3).
If this information is accurate, and there is no reason to doubt its validity, then the celebration of tashlich needs to be seriously rethought by participating Messianics. The only "allusion" to tashlich that I can find in my reading of Nehemiah 8:1-3 is the mention of a "Water Gate". Does the mere mention of water mean that anyone was throwing a rock or piece of bread into the water as a means of symbolizing the abolishment of sin? To extract the tashlich ceremony out of Nehemiah 8:1-3 semes to be stretching the credibility of the word "allusion" to its breaking point. The fact that Rabbi Miller admits that the Talmud does not mention or even allude to the ceremony of tashlich should give caution to seeing tashlich in Nehemiah 8. It would seem that when all the evidence is weighed that tashlich is founded in the Zohar of the 16th century and the Zohar is the clear foundation for Kabbalah ie Jewish Mysticism.
I would encourage all Messianics to forgo the celebration of tashlich since it is not required by Torah, does not find encouragement/mention in the Talmud, and is not clearly displayed in the prophetic sections of the Hebrew Scripture. Those that have abandoned Christmas and Easter on the argument that these "holidays" are pagan in origin should be consistent in rejecting tashlich on the basis of its roots in the mystical soil of Kabbalah.