How can i see/log all requests coming from a registration form (not from the UI)?

You could use “save_post” hook to capture all submit within WP, for ex:

function just_got_you( $post_id ) {
    if ($post_id != '123')
        return;

    << do your logging stuff here >>

}
add_action( 'save_post', 'just_got_you', 10, 2 );

Change 123 to your page/post id. More reference here Save Post Hook