The only way that I can see to make it a little more provably fair to investors is making a system of distributed server seeds, and giving access to each new server seed to a different big investor. That way you can even distribute the risk of holding everything on a single hosting service. So on each roll the central server asks to each seed server what's the hash for that roll of that user, and the seed servers answer with a corresponding hash which is then used to roll the final number.

Main drawbacks would be a lot of added complexity, maybe increased unreliability if you need all the servers up all the time, probably a cost/benefit ratio too high...