The following does not answer the historical aspect of the question directly, but it provides background suggesting that (1.) the circumstances during the diaspora seem to have frequently (if not usually) qualified as "times of oppression", even not during the most acute tragedies of Jewish history, and (2.) the fasts under those circumstances would not have to be declared as equivalent to Tish'a B'Av; they would automatically be so according to the Ramban, and individuals wishing to follow his opinion would conduct themselves stringently during difficult times in Jewish history.1
The Ramban (Toras HaAdam, שער האבל - ענין אבלות ישנה) is understood in different ways. The Bach (OC 550) seems to understand the Ramban as saying that, during some period in Talmudic times (e.g. R' Y'huda HaNasi's time) things were copacetic enough to be in the middle category of neither oppression nor "peace"2 (see Rosh HaShana 18b), but that it was a time of oppression in the time of the Ramban (and the Tur). The Bach concludes that the Ramban holds that one must observe the original requirements of the fast whenever it is a time of oppression.3 The Beit Yosef (OC 550) likewise indicates that the Ramban holds this:
שאע״פ שהם בטלים באותו זמן, אם יהיה שמד חס ושלום יחזרו
R' Yehuda Leib Chanales4 (Vayigash Y'huda OC 550:1) seems to say that the Tish'a B'Av-type requirements were permanently uprooted by communal practice during a period when there was neither peace (since the Beis HaMikdash was not standing) nor serious oppression. However, a resurgence of oppression makes it appropriate (though not required) to observe the original requirements that had originally been absolutely mandatory (by divrei kabbala) during times of oppression.
The Chafetz Chayim appears to take the former approach, and describes his era as a time of oppression (Sha'ar HaTziyun, OC 550:9). He also mentions that the Vilna Gaon (Bei'ur HaGra OC 550:2, s.v. tzomos hallalu) approves of the Ramban's approach. However, it seems to me (IMHO) that the Gra only approves of Ramban's approach regarding the basic mechanics of the prophetic institution of those fast days (see Z'charya 8:19), namely, that the prophets instituted all four fasts with the same types of requirements as those of a biblical ta'anis tzibur ("וכל דתקון נביאים כעין דאורייתא תיקון") but that there was an option to reject these requirements during a period of reprieve ("אין גזרת המלכות ואין שלום" - Rosh HaShana 18b) for all those fasts except for Tish'a B'Av. The Vilna Gaon concludes that R' Y'huda HaNasi exercised this option and uprooted these requirements except for the daytime fasting (see M'gilla 5a-b), and the Vilna Gaon's wording seems to indicate that he would hold that these requirements could not return during a time of oppression (though he says this less explicitly than does the Bach, see fn. 3 below).
In any case, the Ramban did not seem to hold that anyone would have to declare that it was a "time of oppression" in order to require fasting, but rather that the return of oppression ("שמד במקום ידוע בישראל") would automatically generate the requirement to observe the full stringencies of the fasts:
כל שכן בדורות הללו שהרי בעונותינו שרבו יש שמד בישראל ואין שלום הילכך חייבין הכל להתענות מדברי קבלה ותקנת נביאים
Judging by the wording used in Sha'ar HaTziyun, that also appears to be the Chafetz Chayim's approach:
דהאידנא מצוי גזירות מן עובדי גלולים ואם כן חל עצם החיוב על הג' צומות כמו על ט' באב לפי דברי הרמב"ן
1 Besides for the Ramban's opinion, individuals wishing to conduct themselves stringently during any era would be encouraged by the opinion of Rashi (Sefer HaPardes, cited by the ShL"aH, Ta'anis, Ner Mitzva §6, paragraph beginning "min hara'ui").
2 Although Rashi interprets a time of peace to mean a time when the Jews are not subject to the dominion of the non-Jews (Rosh HaShana 18b, s.v. sheyeish shalom), the Ramban defines this as referring to a time when the Temple is standing.
3 The Bach then mentions opinions of various rishonim and finally disagrees with the Ramban and concludes that R' Y'huda HaNasi permanently uprooted the divrei kabbala fasts (except for Tish'a B'Av), and they were replaced with our version of those fasts as a minhag Yisra'el. This seems to be the same as the opinion of the Vilna Gaon.
4 d. 1596. R' Y'huda Leib Chanales Lublin was the son of R' Meir Lublin.