The owner of Freebitco.in is also looking for a solution to this... in this thread he explained how he is running his lottery on his site --->
https://bitcointalk.org/index.php?topic=319540.1180In any scenario, a operator has to show what version of the code he/she is running at the moment when the winner is chosen. Some operators show a "Provably fair" code, but
there is no guarantee that they running that code, when they do the draw.
....The person with the perfect solution to this, would make a lot of money, because this solution
is in high demand.