Make it so theme developers can easily edit the layout of the wp-signup.php page by adding actions after get_header() and before get_footer().
This will prevent users from editing core files.
This will have no effect on non-multisite setups.
Can't you use the get_header and get_footer hooks?
Both header.php and footer.php from the active theme are called by wp-signup.php. Add the hooks to the last line of your theme header.php and the first line of your footer.php.
They'd run before any HTML is output'd i believe.
Personally, I'm +1 for a hook to allow plugins to add extra content to the pages.
Do you have any specific requirements? I'm thinking at the start and end of <div class="mu_register"> myself.