Ok, it took a few references but I found out how to add the base Joomla 'Chosen' functionality back to all the fields.
1. Add a hidden field to an unused position (I assume so that the position will not be ignored otherwise)
2. Create a position override according to here:
http://www.seblod.com/resources/manuals/designer/overriding-a-position
In my case it was /templates/seb_one/positions/article/admin/topbody.php
with the content:
<?php
// No Direct Access
defined( '_JEXEC' ) or die;
JHtml::_('formbehavior.chosen','select');
?>
Now I feel I'm back to a good starting point that's like Joomla starts with. Nice system Seblod is so far - cool stuff.
Magestyx