You seem to have intimate knowledge of MtGox's special bitcoin software.  Do you work there?  Can you tell us how they do this manually at MtGox?  It would be simple if they used bitcoind or something, but it can't handle as many transactions and addresses.

Many of the transactions on the list are deposits, btw.  Not withdrawals.  Deposits to an address where the private key was imported to MtGox are handled the same way.  (I assume you all remember the DoS attack when someone imported the private key to the correct horse staple address, trigging thosands of microtransactions which were never going to get confirmed.)  With the price difference nowadays, I assume BTC deposits outnumber withdrawals by a large amount.

There are a few long standing bugs in their wallet code which have led to BTC withdrawal problems many times, and the rumours say Mark wants a real fix this time.