Here is more info on the Chassidishe Rebbe who davened nusach Ashkenaz, courtesy of The YIVO Encyclopedia of Jews in Eastern Europe.
There is a two volume siddur, צלותא באברהם, that was put out according to his nusach, with commentary and explanations, which can be seen at Hebrewbooks.org.
I just took a quick look at it, and indeed, on page six of volume one, it is stated and stressed that he davened in nusach Ashkenaz davka. It seems that what was meant was an Eastern European version of nusach Ashkenaz, but nevertheless that is still remarkable for a Chassidishe Rebbe.
I will, בלי נדר, examine it more later, and maybe there will be more to say then.
Interestingly, it states that while he davened nusach Ashkenaz, his sons departed from his practice and ceased doing so. I have heard of other cases in which fathers, gedolei Torah, who are or were Chassidic or semi-Chassidic (at least as far as I know), in modern times, davened nusach Ashkenaz, but similarly, it seems as if their children didn’t continue their practice in that regard. I don’t know this first hand, so can’t vouch for it with total certainty, but the reports seem credible.
One case is re Rav Shmuel Wosner שליט”א , one of the great poskim, who lives in Eretz Yisroel. I have heard that he davens nusach Ashkenaz, as per his family minhog, but his descendants have become more Chassidish and daven Sfard. Can anyone confirm this?
Another case is re Rav Moshe Bick זצ”ל, a great poseik in the USA, who was נפטר a number of years ago. I have heard that he davened nusach Ashkenaz, and that he was a descendant of Rav Yaakov Emden, whose siddur (the original authentic Ashkenaz version) he reprinted. I believe that his descendants have become more Chassidish and daven Sfard, though I may be (totally or partially) wrong, and I welcome correction.