I think someone has done this before with Bitcoin and miners found ways to manipulate the outcome. You should find the most transparent method possible to do this, because trust is everything in Lottery games like this. I still have my doubts on both Millionairemakers on Reddit and also the Lottery system, built into Freebitco.in.

Provide a chat area for people who are participating to validate that you are playing against actual people and not some automated bots that are pre-configured to enter these games on behalf of the owner to cheat the system. ^hmmmmm^