This is pretty much impossible to do.
I can think of a way to do it:
Take the message typed, run it through text-to-speech, save the resulting audio file, and play it back in an obfuscated manner (Flash, maybe?) that makes it difficult to simply download the file.
Yes, it's horribly resource-intensive, it requires us to sell our souls, and it still leaves the possibility of recording outgoing audio (either with something like Audio Hijack or just by sending the audio out to something's audio in), but it's more or less what he wants!
