“but it’s not the minhag of most chassidim”
Find me one legit chassidic source that claims so. On Zman tfilah they once had a minhag, and it was based on ???? ????? etc.
However, the ones that you meet at the after 10 minyan claiming so are just a few ???? that slept late, not the ones that were occupied with learing a few hours prior to davening
They indeed have no legit heter even al pi chassidus. I will qoute part:
“??????? ?????? ?? ????? ??? ????, ??????? ??????, ??????? ?? ??? ????, ?? ?? ???? ??????, ??? ???? ????? ??? ?????? ?????? ??? ??????, ?? ?? ???? ??? ???? ??? ????? ???? ?????”
(??? ??????? ???)
?”????? ?????”:
???? ?? ????? ?????? ??????. ?? ??: ?????? ?????? ??????. ?? ?????? ????? ???? ???? ?????. (??? ???? ???? ?”? ?’ 119