You're using an existing document, where I'm using a newly created one, that the user is filling out. Well, if all else fails, I can run an agent to email new replies after they're saved in the database. That may be the easiest solution, actually.
I have created a LS agent that creates a new mail memo, and use appendRTItem there. It fires when documents are created or modified, and checks for a flag that I've set in the button on the xPage. The agent composes the email, sends it and resets the flag on the document so it isn't sent again.
Success! Thanks again for all your help, it was invaluable.