It has always been possible to backstab using the staff of the ram, never tried other Q/S though. I've hit 350 points of damage using a kensai/thief with ***** in quarterstaves, **in 2-handed weapon style using the staff of the ram+6 and managing to pull off a grandmastery KAI critical quintuple backstab! He also had 25 STR at the time (from quaffing a potion of storm giant strength and wearing a symbol of Lathander).
Beat that
