BuddyPress.org

At the simplest level, you can use the following:

remove_action( 'bp_init', 'bp_core_wpsignup_redirect' );

This will prevent BP from redirecting away from wp-signup.php.

There are still Register links at various places in the interface. You could try to find them all and replace them, or you could just write a server-level redirect from BP redirect URLs (by default, example.com/register) to wp-signup.php.