I can't think of a better way to get someone with real crypto cojones to fix their crypto missteps. Power to Kim and what he´s doing. 50GB for free is amazing.
The really good hackers are likely out getting paid good money for their work, and won't necessarily have time to poke at this for uncertain reward. There might be sufficient value in the publicity associated with finding flaws (especially while it's getting lots of media attention right now), but a competition/challenge is rarely a good economic choice for the potential entrants.
A bug bounty programme might be useful as a supplement to a rigorous security audit, but the issues discovered so far seem to be things that could have been identified by reasonably competent netsec people, indicating that such an audit either didn't happen, or wasn't acted upon.