Back to the missing Bitcoins and the original question. Identify the Bitcoins and stop them in their tracks. Whether double spent or stolen, mine are mine and if MtGox sent them to someone else that's theft.
You're not a newbie based on the post count... you should know by now that what you suggest is impossible. Even if it was possible, it would not be fair: The "illegal" bitcoins may have been part of hundreds of transactions after leaving Gox. Those transactions may be totally normal trades between people who had no idea that you would later declare their money illegal. Who are you to say those trades are now void?
Who am I? the rightful owner of those Bitcoins.
How would you identify those specific bitcoin as _yours_?
There aren't specific, numbered, bitcoins. Bitcoin is just a ledger system.
Bitcoin that I deposit at an exchange, would most likely, be used on someone else's subsequent withdrawal from that exchange within the hour. The exchange keeps track of what I deposited, internally.
Same way that when you put USD 200 in cash into an ATM to deposit into your savings account, that money isn't collected later in the day, and put into a box in the vault, labelled "nanonano's monies", waiting for you to withdraw it later from your box.
You depend on the bank, and the exchange, to faithfully keep track of how much belongs to who, and if they are wrong, through no fault of the customer, it is on the bank/exchange, to reimburse you. It would be completely wrong to claw back monies from a 3rd party who had done nothing wrong, depriving them of _their_ monies, so you could have yours.
=squeak=
Every Bitcoin has a history. If MtGox did not assign specific Bitcoins to my MtGox Bitcoin holdings, it at least has a history of what Bitcoins were double spent or stolen. Identifying those that were stolen and freezing those so no more transactions can be conducted with those is what's necessary. And yes, if a 3rd party bought those stolen coins the third party would lose not the rightful owner, MtGox and by extension myself.