You can’t daven to the Rebbe. But you can ask brochos from the Rebbe for anything, as many chassidim do, not just Chabad. And today, even litvishe people by their gedolim / Roshei Yeshivos.
Sure you can ask for a brachah – from anyone. Al t’hi birchas hedyot kalah b’einecha. But if all you do is ask for brachos, why do need the whole shtikl about mimutzah hamafsik vs. mimutzah hamechaber?