Another possibility would be to embed a text message in the transaction. There's no provision in the GUI for it, but the source code allows for it (it's just not implemented at the moment).
I did not know that you can send messages with your transactions.
As Gavin said, you can't. But I didn't say you could - I said the source code supports it. Subtle but important distinction. It would require modifications to pass messages in the prerequisite script passed to SignSignature. And, of course, modifications to see what's there.