Keep in mind that any reveal you have to explain will be anti-climactic.
Most people can't identify notes by sound. So you could literally play any notes and claim they were whatever letters, and the majority of the audience will have no way to know if you're lying or not.
So you'd have to be sure you structured it in such a way that you are allowing the audience to verify. The thing that pops to mind right away is one of those tuning devices that identify the note it 'hears'. The way I'm seeing it, early in the show maybe say, "This is a prediction." then play C-A-G-E. Most people will have no idea what that means. Later, maybe during a book test, force "CAGE". Do the book test reveal and then say, "Remember when I made my prediction? You there, stand here and look at this device ... " Play the 'tune' again, quickly, "That was it, remember? Now .. I'll play it slowly. Please announce the notes as they come up on that display..."
Changes the book test to a prediction, so be aware of that while framing that, but it could work.