In an interview, Rabbi Heschel Greenberg answered to that question:
"Rav Herschprung of Montreal, after Gimmel Tammuz, wrote that the belief that the Rebbe is Moshiach is a hundred percent valid, and - I don’t have the letter in front of me, but - it’s even stronger than Rav Aaron Soloveitchik’s [sic] letter.
(I'm not sure whether he means that Rav Herschprung himself believes that the Rebbe is Moshiach, or at least that it is a valid belief.)
"Rav Eliyahu Shmerler, the Sanzer rosh yeshiva, signed a document saying that he believes now [i.e., after Gimmel Tammuz] that the Rebbe is Moshiach. Rav Ovadiah Yosef’s son, Rav Yaakov Yosef, who’s not a Lubavitcher but a Sephardic gadol, signed a document saying that the Rebbe is Moshiach. And there are many others.