I reckon a long/medium travel hard tail should be the bike of choice. I have ridden up there a fair bit over the past couple of years and it is pretty well suited to a good hard tail. I use a Genesis Alpitude which has survived the abuse the quarry can give a bike very well.
It can be tough on tyres and chain rings, highrollers seem to work for me pretty well up there. I see a lot of people with punctures in the quarry and find loads of discarded inner tubes (hate it when people just drop their rubbish like that!). Tyre choice might be as important as choice of bike. Brownbacks race last week Tom Bell had a good 5 min lead and then he got a flat and had to drop out, don’t know what tyre’s he was using.
Also see a lot of people snapping chains during the races, short steep downs followed by short steep ups seem to result in lots of gear mashing for some.
I think the loop we have planned for the xc race would benefit from an fancy up and down seat post type thing. I don’t use one myself but I could see being able to get the seat out of the way quickly for the downs and then back up for the climbs being useful. But would it be worth the extra weight?
End of the day it’s probably much more down to the rider than the bike, I can think of loads of people who could beat me on a lap round the quarry riding a Raleigh chopper with a flat tyre and one hand tied to their crank.