There is no chain of title from the depositor to the current owner of a coin. MtGox being off blockchain and pooled funds makes sure of that. You can trace a particular "coin" back to MtGox, but the trace stops there. You can't trace back further than that.
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application of demo dat rule is not a forgone conclusion when it comes to bitcoins. It doesn't apply to legal tender and it also doesn't apply to bearer instruments (i.e. casino chip), or negotiable instruments (i.e. a check). It is at least plausible a judge would rule that bitcoins are more like those exceptions than real property. Until we see a court case we won't know for sure. I am not a judge so what I think matters little but a bitcoin has more in common with a casino chip (a bearer instrument) then it does with a car (which is a unique specific piece of real property).
You have chain if title from the victim to MtGox. With records from MtGox you will also be able to identify the stolen coins that left MtGox. That is a sufficiently strong chain of title for a lawsuit.
Your assumption that the courts will rule that bitcoin should be treated like cash rather then a commodity is in my opinion foolishly optimistic. I have no horse in this race (not a MtGox customer). But at the very least this creates huge legal uncertainties for bitcoin.