For anyone interested, we added a promotion that pays 25$ in Bitcoin per day (exclusively for Bitcointalk.org members) if you maintain the top 1 position on all-time leaderboard.
From what I've seen (in the past) on online casino's: the user with highest total profit barely changes. Once someone hits it, he can keep that position for a very long time.
Once someone has a negative profit, he has more to make up for and will likely be out of this race. A daily or weekly race for a Leaderboard might attract more users, as they're more likely to win this.
Daily leaderboard is most suitable for this type of competition. But OP said that there isn't enough activity of the users. A forever leaderboard will not attract new people as it will be tough to beat the top profit makers. Because they have low betting limits and maximum 5 bets can be kept open at a single time. Giving prize to the all time highest wager maker will be reasonable. But the profit maker contest leaderboard should get cleaned each day. BTW, I will opt in the contest if OP change it to the daily one.