You can open the Game.pak file (in the '..\Divinity - Original Sin\Data' folder) in a text editor that can handle the size (Wordpad, etc) and search for your message.
Don't save, of course; if you wanted to make any changes, you'd have to use a hex editor.

If you can not find it, email supportdos@larian.com, with the email address you used when you backed the kickstarter and/or have linked to your Vault account, and what the message should be.