Certain extra risks, like "double or nothing"? You can bit for a slot, with a 50% chance of getting it, and 50% chance for getting the ad slot for two weeks. Something like that.
What did you do for the other big events (100th ad round I assume)?
How about having more than the normal number of ad slots.
I think it would require changing some coding and doing it just for a week wouldn't be worth taking all the pain.
But if possible a additional ad slot after every 200th post and its multiplier would be a good incentive for advertisers to bid high for this week.
Of course, you could automate it, but I'm pretty sure Theymos manually has to upload ads every week, and it says on the thread that it could be longer than 7 days, so it's not automated. In fact, it would be easier for him.