You should take a look at our instant gamble. We have already built this game and it works rather well: https://www.bit777.com/support/instant_gamble.aspx . We have had this game for the past 2 months. It can't be cheated by the method above, because it also uses the 2nd hash generated by blockchain.info api when rerouting the transaction towards the final address.
How can that possibly be provably fair? The outgoing transaction contains the winnings which means the odds have already been computed, so you can't possibly use that hash to compute the random number.
