So try this: instead of saying, "Memberships $100 until date X," announce something like, "The first 1000 memberships will cost $100; the next 1000 will cost $120, the next $140," and so on. You'd have to include provision for "ties," with people who joined before you could actually announce that the "bucket" had filled, but that's not a huge issue in my mind. And yes, there's an existing provision with a time-based price (the 90-day rule), and you'd still have to abide by that, but after the 90 days is up, the supporting members who didn't convert would be subject to whatever the current pricing bucket is.
A "bucket" approach would very possibly give the people planning the event a bit more budgetary certainty, although they would still have the uncertainty of total attendance projection. And it would, in my opinion, increase the pressure on people to join sooner, since they would know that if they hesitated, the "bucket" might fill up before they got around to joining.
There is the other side of the coin, though: The most price-sensitive people are the people who have never attended, and they're the ones I'm personally most interested in reaching. After all, I don't have to sell Worldcon to people who attend them regularly; they already know what it is. I've said this before, but if I could find a way to offer a large discount to first-time attendees, I'd do it. It just wouldn't fly politically, however, as the current system is geared to privilege the longest-term members and they're the ones you dare not antagonize.