There are some attacks against this.
For one, the Ponzi operator can join your system, and withdraw everything a moment before he shuts down.
Thats the beauty of running this on an exchange, there is no out, there is no risk of a bank run. Ask OBSI.HRPT investors, the only way out is selling your bonds (/shares) at market value.
So you require some sort of lock-in mechanism that's spread out over time. Still, you need to measure the Ponzi's actual size, or else someone can burn you out by using massively inflated numbers. They never offer internal bookkeeping, so that's also an annoying task.
Indeed, this was already brought up and answered. Again its not a problem on an exchange where you can find this data, but it would be a problem for an off exchange ponzi.
As Pirateat40 said, the market for his kind of operation will be saturated after he leaves. I don't see a big BTC Ponzi coming up any time soon, now that everyone is more skeptical.
Lets hope none ever gets remotely as big as pirate again, but there is no shortage of new and old ponzi's. Have you checked this subforum recently?