If anyone can think of a better way of calculating "how lucky" the players have been (individually, and as a whole) I'd love to hear it!
Somehow relate the luck to the wagered amount, so a single win with 1 satoshi on 0.0001% does not get the site's luck up from 50% to 250%.
I think the statistic you're describing is "profit". Or maybe "profit/wagered". If you take stake into account then the players luck is directly proportional to his profit/loss. I was aiming for something which was independent of stake, so we can see things like "I had a really unlucky set of rolls, but my martingale betting still let me make a profit". And such.