These gimmick promotions pop up all the time with no one ever receiving the bounty. Could you you possible put the reserve funds in escrow with a reputable 3rd party? It would bring some legitimacy to your great idea.
I do not have anything to gain personally from this, so I don't see what the problem is. In any case, I am not opposed to the idea of putting the funds in escrow, but I really don't want to put in the time and effort to figure out how to do that. If someone can set this up and convince me that the escrow service is legitimate, I will do as you suggest. Although I was kind of looking forward to sending the funds to the winners myself...