The addresses of the unsent and unrecoverable coins are easy to spot on the blockchain, right? So just issue a hard-forking update that makes those coins invalid. So long as those addresses are never used again, nothing will be different; if they 'mysteriously' come back to life, those transactions will never be confirmed because the vast majority of the mining power will be with the updated client which sees them as invalid and doesn't include them in the next mined block. It doesn't even require that all users upgrade.