Is there a way to input different rel=author and rel=publisher attributes for a WP blog w/ multiple authors?

Well, you can try my plugin ;), but almost every SEO plugin includes this functionality now, and there are several plugins which are kind of dedicated to it.

If you want to code it yourself for whatever reason, all you need is basically to add a field in the user profile admin page in which the user google profile url, and use the data in the header

profile related code

add_action( 'show_user_profile', 'wase84176_user_fields' );
add_action( 'edit_user_profile', 'wase84176_user_fields' );

function wase84176_user_fields($user) {
  here should go some wrapper HTML to make it display nice 
  Enter your google profile URL <input id="wase84176_user_url" type="checkbox" value="<?php echo esc_attr(get_user_meta($user->id,'wase84176_user_url',true)?>" name="wase84176_user_url"> 
}

add_action( 'personal_options_update', 'wase84176_user_fields_save' );
add_action( 'edit_user_profile_update', 'wase84176_user_fields_save' ); 

function wase84176_user_fields_save( $user_id ) {

    if ( !current_user_can( 'edit_user', $user_id ) )
        return false;
    $opts = array();
    if (isset($_POST['wase84176_user_url']))
        update_user_meta( $user_id, 'wase84176_user_url', trim($_POST['wase84176_user_url']) );
    }
}

add the link in the header.

add_action('wp_head','wase84176_user_head');

function wase84176_user_head() {
  global $post;

  if (is_singular()) { // an author can reliable be set only for single pages
    $m = get_user_meta($post->post_author, 'wase84176_user_url',true);
    if ($m) {
        $data = get_user_meta($post->post_author,'wase84176_user_url',true);
        echo '<link href="'.esc_attr($data).'" rel="author">';
    }
  }
  if (is_author()) { // and on the author posts page
    $m = get_user_meta($post->post_author, 'wase84176_user_url',true);
    if ($m) {
        $data = get_user_meta($post->post_author,'wase84176_user_url',true);
        echo '<link href="'.esc_url($data).'" rel="me">';
    }
  }
}

Will not go into details why single pages get rel=”author” while the author page get rel=”me”, I just felt it is more in line with the spec but the way google authorship works any of them should work