I am pretty skeptical about that and I still don't think it's possible for a fast game like dice. But yeh, if you don't publish "how" then we cannot check it obviously (:
This one
http://satoshinonce.com/ (don't know if this is still running fine) appears provably fair to an investor?
I don't think it is impossible, it will just be slow. We need one variable that is not determined by the server or the client and cannot be influenced by either but becomes known soon-after like a block hash?