It doesn't necessarily need to be open source, it could just be shared with a trusted group of coders for review. Then the code on the server could be hashed regularly and that hash compared to the hash of the code that had been reviewed. Any code changes would quickly be obvious to anyone who bothered to check.

Maybe what's really needed is some kind of trusted service/group who would devote time to reviewing/certifying the code behind these online casino operations...?