Mathematically, he is still in advantage, because house edge is smaller than his 24 hours wager. Let say he wagers 1 BTC in the last 24h, and the site wins 0.01BTC, he still earn 0.023 BTC (0.001 * 24h)..
.024 - .01 = .014, not .023. But you are correct in that as long as he is wagering .1 or less for every hour that he wins the leaderboard, it is a profitable play.
Now the question is, how high is he willing to go? I guess that depends on how long you are guaranteeing the promo will last. Will he take a -ev play in the short term in hopes that he will outlast any potential competitors? That's the downside to this game...all it takes it one more person who is willing to lose money in order to be on top and both players will lose. The only way that he wins is if nobody else even tries...which makes it so that people have less of an incentive to play on your site.