In the 2005/5765 Kehot edition of סליחות על־פי מנהג חב״ד/Slichos appears part of a letter by the last Lubavitcher rebbe with no date on it. It reads, in part:
לא שמעתי הוראה בסדר סליחות ליום זה (וכן — אם לאמרם באשמורת הבוקר, או בתפלה בסדר התחנון)
In my own translation:
I haven't heard any direction about the order of s'lichos for today (Tzom G'dalya) (nor, indeed, whether to say them in the wee hours of the morning or during tachanun during the prayer service).
How does it happen that in two-hundred-odd years of Lubavitch custom there is no established custom regarding whether to say s'lichos in the early morning or during shacharis? Did the rebbe not see what his father-in-law (the previous rebbe) did? Is there no record of what the previous rebbes did? of what the community of chasidim did? How can this be?