Interesting from a game theory perspective. In that context it seems like the discussion so far is making a fundamental error in figuring out how it would work, by assuming that the escrow amount would be some small fraction of the total recovery amount.

The correct way to do it would be to make it so the escrow amount is the same as the amount to be recovered or even more, and for the service to return that full amount plus the keys to the account upon a successful challenge.

So if you're the account holder, and you're sure you are, and sure you've just lost the keys, bricked the hard drive, whatever, you know for sure that you're going to win the challenge. The only issue is being forced to come up with the initial funds to make the challenge, which isn't too hard to do.

But for the speculative challenger that everyone's hypothesizing, they'd have to risk a substantial amount of money for a relatively small payoff. The larger the escrow amount the more unfavorable the bet looks to a challenger, it wouldn't take much to make the entire concept of a nefarious challenger obviously unworkable.