Not a very solid idea I'm afraid and a very time consuming one at that which I'm not fully certain the staff should put their time around the boards and site into.
Trying to keep this post somewhat short, I can see that we as a staff would need to take many measures just to avoid having people that cheat on these things, and a things with any hunting/seek thing is that people tend to help each other and just give away the positions just like any other contest in such a manner.
Also count in the time just randomly placing all these things around the board, also save down the locations and eventually remove them all the the contest is done and settled, which threads would we post these in if not only our own? What about checking for pemission for which threads we could post this in? We cannot make any new ones since they'd end up bumped to front, we couldn't lock them since it would make searching easier in some cases, users could very easily bump the threads too. And then we need to have at least one staffer sitting and collecting these and reviewing them.
There's probably more things to this, but I'm afraid I don't quite feel it and how it would work in a way that wouldn't turn out messy. It's a great idea and maybe it can be adapted some way, some how after some thinking, but as it stands right now I'm afraid not.