Well, only if the people seeing it knew the decryption algorithm. You could be extra tricky and reverse the meanings if you wanted to get the competition in trouble ;)

Like I say, I'm sceptical, but am quite prepared to entertain it for a moment or two, even if only just to find the flaws. That's a large part of the fun innit? It's a step above some of those ones Morris listed, like the one about adders nests...