The Tur (Orach Chayim 580), quoting the Bahag (Laws of the Ninth of Av), gives a list of days on which to fast. This list is copied or mentioned by some other rishonim (as quoted in the footnote to the Bahag: Eshkol part 2 p. 9, Shibolei Haleket 278, Tanya 62, Seder Rav Amram 34a, part 2 74a (?), Machzor Vitri 271, Kol Bo 63).
Some sources (e.g. Magen Avraham) say the list comes from Megilat Ta'anit, but this appears to be incorrect. Although they are written in the style of Megilat Ta'anit, all the dates are in Hebrew, not Aramaic as Megilat Ta'anit is, and the response of the ge'onim (quoted in the Shibolei Haleket and Tanya) also implies that Megilat Ta'anit is separate from it. And the death of Tanna'im is also mentioned in the list, making the second century the earliest possible date, at least for its final form.
The response from the ge'onim that shows that they didn't know who decreed these fasts. The Beit Yosef says about them that he never saw anyone observing any of the fasts, and expresses surprise that some of them are on Rosh Chodesh.
My question is: Apart from the question to the ge'onim, is there any sign that these fasts were actually observed at some earlier point (not necessarily the entire list, but excluding 17 Tamuz, 9 Av, 3 Tishrei, 10 Tevet)? Are any of the days mentioned as having been a day on which someone fasted in any of the sources, especially before the time of the ge'onim?
I ask because Josephus (Antiquities 14.4.3) mentions a fixed fast in the third month (apparently in Sivan). Some (William Whiston in the footnote there, and this book I found through searching) suggest dates from this list as the fast he mentions. However, if the dates never appears until the ge'onim, it seems unlikely he was referring to them.