I'm imagining a series of clues (think National Treasure), but not too obvious. If it's to be a world-wide hunt, it must therefore be difficult to crack. I don't want to hide the private key on a website or anything, but rather have the pieces be freely found in the public. So perhaps
could represent the 233rd page of some book and so on.
Would love feedback and ideas.
Why not ask in a geocaching forum? They are experts in creating treasure hunts.