Haha, well you probably could use some really complicated maths to get it bang on, but I have no idea how to do any that.
I think it'd be fair if for each session the average score of all the drifters' best run is made. Then the single run closest to this average score as well as the upper and lower quartiles (halfway between the middle and highest/lowest run) are then judged by the head judge. The percentage difference between the runs would then be used to either bump all the scores in that group up or down.
For example the best scores of one session are as follows.
Drifter 1: 19.00
Drifter 2: 17.00
Drifter 3: 15.00
Drifter 4: 13.00
Drifter 5: 11.00
Drifter 6: 9.00
Drifter 7: 7.00
Drifter 8: 3.00
The average score of the session was 11.75, so Drifter 5's run will be moderated. Then Drifter 3 and Drifter 7 will too, as the Upper and Lower Quartiles are 15.375 and 7.375 respectively. Then, if the Head Judge judges that the runs are in fact worth 6.00, 10.00 and 14.00, all the scores in the group will then be lowered by whatever 10%, as that is the average percentage between the two judges.
Obviously this is a very basic example and in real life the scores probably won't be anythin like this. However, I do believe it's a fair way of doing things without you having to sit through hours of replays!
I've probably made it out to be more complicated that it actually is, but I'd be happy to crunch some numbers for you. In fact, an Excell spreadsheet could probably be made to do everything for you.