I’ve heard that Rav Shlomo Yitzchak (Rashi) would modestly sign his letters with his initials (shin-yud), and that others would then use that convention when referring to him, first respectfully putting a reish (rav) in front, making “Rashi.” So it’s highly likely that he was referred to as “Rashi” even when he was alive. As for others with acronyms, I have no idea.
