In this lecture Dr. Secunda postulates a connection between the Zoroastrian Kushti belt and the external belt some (mainly Hasidic) Jews wear during prayer called a gartel. The possible reason for this postulation is that, unlike the basic halachic requirement the reason to divide between the heart and the genitals, the gartel is worn to divide between the upper more spiritual portion of the body and the lower part which is seen as connected to mundane (sexual) desires. This is the same reason the kushti is worn. I would like to know if there is any other information supporting or opposing this possible connection in Jewish academic or religious (responsa) literature.
No, the Chassidic practice of wearing the gartel, which is properly called Avnet (אבנט) in Hebrew, was not influenced by the Zoroastrian practice.
It is worth pointing out that after listening to the entire linked lecture, the premise of this question, that Dr. Shaya Secunda postulated a connection between the Chassidic practice and the Zoroastrian kushti belt, is not accurate. In the lecture, Dr. Secunda was recounting a discussion between a Babylonian Amorah of the 6th century CE named Amemar and a Zoroastrian leader of Persia.
This discussion can be likened to the religious contests that have been demanded of Jewish leaders over the millennia, like for example the disputations of Ramban in Spain.
In the case of Amemar, a story was recounted of a meeting between a Naggid of the Jewish community in Persia named Huna bar Natan and a Persian King named Yazdegerd II. Huna bar Natan was primarily a political leader and not as knowledgable of proper Jewish behavior. When King Yazdegerd met with Huna bar Natan, he adjusted the placement of the Avnet worn by Huna bar Natan explaining to him that Jews are a nation of Priests and therefore he should wear his Avnet where Priests wear it and not too low (meaning at or below the waist) or too high (meaning at the armpits or higher). The King went on to explain to Huna bar Natan that this belt separates between the holy, upper portion of the body and profane, lower half, like is taught by the Zoroastrians in regard to the Kushti belt.
Dr. Secunda explained that this is based upon the idea from Shemot 19:6 which recounts what G-d told Moshe to tell to the Jewish people:
וְאַתֶּ֧ם תִּהְיוּ־לִ֛י מַמְלֶ֥כֶת כֹּהֲנִ֖ים וְג֣וֹי קָד֑וֹשׁ אֵ֚לֶּה הַדְּבָרִ֔ים אֲשֶׁ֥ר תְּדַבֵּ֖ר אֶל־בְּנֵ֥י יִשְׂרָאֵֽל׃
Dr. Secunda went on to explain that Kohanim had as part of the Priestly garments a belt called Avnet and that most Jews today do not have a special belt but rely upon the belt in their pants.
To this, a person in the audience commented that the Rabbi in her synagogue wore a special belt during prayers. And Dr. Secunda commented that today, among Jews, this practice is mostly limited to Chassidic practice. This last comment is not accurate. The Avnet is worn by Mekubbalim, Chassidim and many in the Sephardic and Mizrachi communities.
He explained that the Zoroastrian speaking to the Amorah, Amemar tried to equate the function of the Kushti belt of the Zoroastrians to the function of the Avnet used by the Kohanim in the Temple service, meaning to separate the holy from the profane. That Zoroastrians believe in a kind of dualism with one deity ruling over the holy and one ruling over the profane. The Kushti belt marks the territorial boundary between the two. And in this way, showing how the beliefs are similar, the Zoroastrian would proselytize to the Jew. Dr. Secunda commented that as far as he was aware the belief that the belt separates the holy from the profane appears to be similar in both Jewish teaching and Zoroastrian teaching like is recounted in the story. This last comment is a common misconception found among many Ashkenazic communities.
The Chassidic practice today is based upon the teaching found in the Torah concerning the Priestly garments that are required as part of the avodah of a Kohen in Shemot 28:4 and in VaYikra 8:13. It has nothing to do with influences of Zoroastrianism.
From the most obvious perspective, this explicit detail from the written Torah concerning the Avnet was commanded by G-d to Moshe shortly following the exodus from Egypt. The exodus occurred in the year 2448 of the Jewish calendar. Moshe Rabbeinu was 80 years old at that time and this was at the close of the 14th century BCE in 1312 BCE.
The prophet Zoroaster (Zarathrustra) who was the originator of Zoroastrian worship lived according to most historians in the 6th or 7th century BCE. The earliest reliable dates mentioned on Wikipedia say he may have lived as early as the 10th century BCE or around 1000 BCE.
Chronologically, the commandment from the Torah and the teachings associated with it preceded Zoroastrianism by more than 3 centuries.
But more important than the chronological precedence, is that the Jewish teaching concerning the Avnet is precisely the opposite of the viewpoint attributed to Zoroastrianism.
The Avnet is distinct among the garments of the Kohen in that it marks not a point of separation and division, but rather demonstrates the unity of everything as is found in ערכים חבד No. 1 5731, page 58.
ענינו הרוחני של אבנט הוא דוגמת פעולתו בגשמיות:
> אבנט אינו עשוי לכסות ולהגן על גוף האדם (כמו לבוש), אלא לחבר ולהדק את הלבושים שיהיו מהודקים היטב לגוף האדם. וכן ברוחניות: האבנט אינו כשאר הבגדי כהונה, שהם בחינת מקיפים פרטים להכחות פנימיים (מצנפת מקיף לראש, חב״ד. כתונת — לגוף, חג״ת. ומכנסים — לרגלים, נה״י), וענינו אינו אלא לקשר המקיף בהפנימי, כי אין המקיפים מעמידים את הפנימיים, רק כשהם בקירוב אליהם.
הכח שיש באבנט לקשר את המקיפים פרטים בהפניניים הוא לפי ששרשו למעלה מהם, שכל הלבושים שרשם מבחי׳ מקיף דחי׳ ואבנט ממקיף דיחידה, ולכן ביכולתו לקשרם.
It goes on to explain that, according to the teachings of Chassidut, the correct place to tie the Avnet (the gartel) is at the beginning of the breast (meaning the base of the sternum, opposite the heart) and not like those of us who tie it around the stomach. This last detail is because the heart is associated with the King, meaning G-d's Kingship like is found in Mishnah Torah, Hilchot Melachim 2:1 and 2:5.
This concept of a unique element from the external, surrounding structures which reveals the unity of everything is again repeated in the construction of the Mishkan described in the Torah in regard to the HaBriach HaTichon (הבריח התיכן) in Shemot 26:28. This is the bar which ran through the center of every plank which made up the walls of the Mishkan.
And this is part of the essential service of every Jew in being a nation of Kohanim, namely to reveal that G-d is one in the Heavens above and on the earth below. There is no other.