Quite correct Nemesis ... it has been used on the Three Castles event in June a few times. It cannot be used as a full timed blast, there has to be some stop astride lines and cone chicanes to ensure the speeds do not get dangerously high. I have marshalled at the top of the hilll and it's a wonderful noise as some of the old cars negotiate the tricky bends.
We used to get about 20 cars to the top then stop the timed runs to convoy them down, then run the next batch. We had radio link between the top and bottom to monitor all the cars. Most years we had glorious weather although there was one very damp one I recall, not a nice place to be then and not good for the open cars occupants!
In my role as Chief Marshal for four years on the event I have driven up there many times, but not at competitive speed!
Another hillclimb used was the road up to the Marchlyn Lake above Llanberis, choice of two courses there, both equally challenging!