It would be really great if someone could post a quick how-to so that everyone can check that they can do this before the shut-down, otherwise there will likely be many tears.
I am sadly too busy to do this, but am just throwing this out there as an invaluable suggestion...
I found this thread which breaks down the concept of signing a message. Unfortunately, the term "sign a message" is a little counter-intuitive. It should read "create a message signature". In essence, you use Bitcoin-QT or Blockchain.info or whatever client to generate a signature key for a specific BTC address AND the message associated with that address as it is typed in the message window. THEN you copy that information to an email, PM or whatever and send it. You do not send the message you generated the signature for from the client it was created in.
This thread is specific to blockchain.info but the concept is the same across all clients as it is a function of Bitcoin itself. Also - you can change the address associated with your bonds (shares or whatever) in Bitfunder. If you are one of those people who used an invalid address you should perhaps consider fixing it by editing your settings in Bitfunder.
https://bitcointalksearch.org/topic/help-how-do-i-sign-a-message-using-my-blockchaininfo-address-180068
But it's secure due to the fact that if the message is changed it can't be verified...