I don't think they lost 740,000BTC in the sense of "they had it" versus maybe they were selling Bitcoins they never really had in the first place. :-/
Very interesting.
I think you've come up with just about the only way that 740,000BTC number could in any way be connected to reality. In other words, they've been fractional reserving and 740,000 is their reserve ratio times their actual on-chain loss. If they were levered 10:1 the numbers start to get believable.
It would also provide a believable explanation for why they knowingly and intentionally drew down their cold wallet to a 0 balance (the bullion banks are doing the same thing right now).
So it's more Madoff than Corzine.