I've been hosting and playing Halo for a long time. I thought it would be nice to have an auto-message so I looked around on Google. The only thing I found after ten mins was this, your post. Shortly after I found something that did the trick. Seeing that your post wasn't from 2007... I wanted to tell you I found something. I know how frustrating it can be when absolutely no one knows what your talking about.
I got an account here just to answer your question! XD
Here's what I found.http://www.gamefront.com/files/12456733/Halo-Trial-Auto-Typer-/-Auto-Host/
It doesn't let you say anything once you turn it on but if your idle, its grate.
You can also set how many mins before the message repeats.
It's not grate, but its useful.
Hope its what you needed. :)
(This is my very first forum post. lol)