How about a flashing cautionary sign that says "DANGER: BLIND HILL" or "DANGER: HIDDEN DRIVEWAYS." When I drive in rural areas with twisty, hilly areas, those signs are quite common. I fear that having a stop sign at the top would cause rear-ending accidents.
In St. Paul, MN there is a famous and steep hill called Ramsay Hill which is in a VERY high traffic area. It has a 3-way stop, and a "Stop Ahead" sign coming up the hill to warn of it. Still, there are the occassional accidents.
ETA: Yes, a manual transmission on a steep hill is quite nerve-wracking, but it is something that can be learned. I mastered starting from a stop on hills in my stick-shift Jetta when I was 17, but I could have caused LOTS of accidents during my learning process as I rolled back, or stalled.