One of the reasons that the World Land Trust does not go after celebs and celeb endorsement is clear if you read this article from the Telegraph:
We have quite a few famous people connected with the WLT, but their connection is always sincere, genuine and relevant. Famous people have done something to be famous for. Celebs are more often more famous for being famous than for what they have actually done.
Unfortunately the story exemplified by Bono above is far from unique. All too often high profile fundraising goes hand in hand with high overheads. That is why the WLT relies on word of mouth, web presence and similar ways of communication. So you, my handful of cyber-supporters are worth a dozen celebs. Keep up the good work and spread the message.