Hook and/or CA to customize checkout completion message

I'd like to be able to use a different checkout completion message depending on the presence or absence of a particular item in the current order. Unless I'm missing something, there's currently no way to do this. I tried using a PHP filter but the order object wasn't available within the eval context.

If someone has some pointers on how to write a CA or a hook to do this, I might have time for that.