Well, I think that I might have read that they intend to kep the downloads and thus keys available for as long as possible, and for the statistics to remain real-time and updateable, they must store how much each user paid.
The data is probably there, so it may be possible for the people who organized the bundle to set up a system(maybe a one way key to unique code mapping?) that would allow developers to see if a user purchased without them giving away their download key...
Wolfire has enough information to prove that you purchased(proof: downloads still available, statistics claim to be real-time, from purchased screen you can update your individual stats, the key resender uses your email address), so if they wanted to, they could possibly theoretically have a system where you take a URL from here, it asks you to type in your email, you do, it sends you an email containing a key generated according to specifications, you take that key to aquaria, if the key meets the specifications(and it could possibly encode payment quantity into categories like <5$,5$-10$,>10$) then they could confirm your purchase...
However, all of that is only based on my theories of how much data a wolfire kept, and in the theoretical situation where both wolfire and bit blot wanted to set up such a system...
