I wouldn't advertise rewards. But it would be nice like at the end of 2013 if Riot gave surprise rewards to certain people who did well in tribunal.
Wouldn't give anyone who wasn't already doing tribunal a reason to start because the reward is random and not guaranteed the next year and is infrequent.
I don't need a reward to keep me voting but a pat on the back is always nice.
The only way you can "reward" is to make it not a real "reward" (which is giving a positive reinforcement for an action).
This means whatever "reward" has to be done at a completely random time and be explicitly one time (at least said to be one time).
Then, going forward, even if you have some poeple doing tribunal regularly hoping for a reward, that isn't so bad, but most wouldn't put in the months of effort or what not, to do it without another reason (whether its trolling or making game better or whatever)