I like the idea. If the message is going to be bombarded to the whole world for a week, it needs to be something proportionately amazing. Though most quotes that are worth remembering come from celebs (increasing the probability that the message will be equally memorable), I think some moderation should be in place. If the message is not that significant, may be, the opportunity goes to the next highest bidder? I am not sure about this. Will it be humiliating for the bidders and discourage them? On the other hand, discouraging one bidder with a low-quality message is better than discouraging the entire world by making them hear it. I initially thought that the bidding results could be kept a secret. If the message by the winner is not great, the next bidder is contacted and asked to submit the video without disclosing the name of the first winner. However, the bidding cannot be kept a secret to maintain transparency of the whole process and avoid the problems that will surface otherwise.
The moderation will also edit the message, while preserving the meaning to make it sound more powerful and impactful, and simultaneously eliminate any phrases/words that may sound discriminating/ unnecessarily vulgar.