So still working on this mod from time to time, but lacking the time to fish it up.
But I wanted to at least show you this update.

I did by accident more or less discover how to change the number of filtered products shown between the brackets.
If you search in file includes/templates/YOUR_TEMPLATE/sideboxes/tpl_dynamic_filter.php.
and change
PHP Code:
htmlspecialchars(html_entity_decode($attributes->fields['quantity'], ENT_QUOTES)) 
to
PHP Code:
htmlspecialchars(html_entity_decode($attributes->fields['flag'], ENT_QUOTES)) 
the number also changes to the number of products left on which to filter.
You can see it in action here