Re: wp-signup.php redirects to registration-slug – WHY???
I just noticed I failed to read you entire question there. Correct, they will target wp-signup.php and you can modify this file name too, if you wish. It is a bit involved though and may I suggest this article if you would like detailed information about how to do so ~ http://wpmu.tripawds.com/2009/12/12/the-ongoing-fight-against-spam-blogs/
keep in mind those instructions are for MU 2.8.6 and if you decide to edit wp-signup.php ~ don’t forget to change it in these places ~ wp-signup.php, wp-login.php,wpmu-settings.php and bp-core/bp-core-templatetags.php