Customize wp-signup.php Message?

I’m trying to alter the text on wp-signup.php, but it seems the only real options for modifying that page are before_signup_form and after_signup_form, and there are no filters on the actual message. I’m trying to replace the stuff like

“By filling out the form below, you can add another site to your account. There is no limit to the number of sites you can have, so create to your heart’s content, but write responsibly!”

with an image, and some of our own text. Does this come down to using jquery to awkwardly replace the text? 🙁