I would think that you could do that, but you'd have to give it a try. Just set up the absolute URL to target ListMail on your server.
One thing to consider would be the opt-in notification, confirmation and error HTML pages that come up when the user submits their info. I'm not sure how that would work since it is coming up on another site.
My guess is that you will have to have those custom pages located on your server.
One other thing - please read Dean's license agreement and see if what you want to do is legit. I assume that it is okay since the application is only on one server, but check that out.
Keep in mind, I'm kind of an amature when it comes to this stuff.
Steve