If I understand you correctly then most custom columns are meant to be just added in the book detail. There's little interest to add other filter by custom columns (like author, serie, tag).
I'll think about that.
Have you tried to change fetch.php like I told you ?
if you don't want to change it manually. Can you try to replace your fetch.php by https://raw.github.com/seblucas/cops/master/fetch.php
And edit your config_local.php to update these 2 config items :
$config['calibre_internal_directory'] = '';
$config['cops_x_accel_redirect'] = "";