Although, thinking about it, maybe you could obscure it by depositing the rewards to an exchange and then sometime later "withdrawing" those funds to the intended recipient. Would be hard to tell the difference between that and normal activity.

I think that would work - it would probably have to be done manually and could be done in such a way that it's done monthly... and perhaps every user is in a monthly league table - where only they can see their own rank. The top 3 or 5 or however many receive the rewards.