I don't think adding a mana lottery would be a good idea in my opinion, as it's very easy to get mana.
For every block you mine, you get one mana. MVP+ get /kit mana, and if you have done the Demeter's rebirth, and have purchased all of the mana workers, you get 3,500 x7 every day. You get mana from vote crates, selling things on /ah and with chest shops. While I do appreciate the idea, I think there is already enough ways to get mana on Olympus, that are very straightforward.
At some point in the not so distant future, a chat scrambler will be added to Olympus (like the one on creative) where the first person to unscramble or type a word, will win a certain amount of mana. So that's another easy way to get mana, and is slightly along the same lines as what you're suggesting, except there is no entry fee.