Save http://brainwallet.org source to an html file on your flash drive w/ some linux live boot distro.  While still online go to the "transactions" section, enter your source address to get your previous transaction history (click "edit inputs" and save to txt file on flash drive). 

Boot live distro on your cold storage machine.  Open brainwallet html in firefox & go to the "transaction section".  Copy in transaction history from your txt file.  Copy in your private key (use bitcoind to get key from wallet.dat if necessary).  Copy destination address and the amount.  I'd recommend specifying a destination address for the remainder (to avoid accidentally giving some miner a huge fee).  I believe you can use the same address for the remainder as the source if you want.  Brainwallet may default to this, but I'm not 100% sure.  Hit re-sign if necessary to produce a raw transaction.  Save that raw transaction back to your txt file.

Now you can copy that raw transaction into http://blockchain.info/pushtx or bitcoind to send the transaction without the online computer ever seeing your private key.

PLEASE TRY THIS WITH A SMALL AMOUNT FIRST.  VERY EASY TO ACCIDENTALLY SEND A LOT OF MONEY TO THE WRONG PERSON - LIKE A MINER.  GOOD LUCK.