[turba] Automatic free/busy urls for kronolith in generated "group" adressbooks

[Johannes Boy]

Hello all,
to ease the pain of managing various calendars for about a dozen users, 
I'd like to make the free/busy information automatically available in 
kronolith. My current approach would be to use the group- based adress book.
However, that adressbook does not contain the free & busy urls and is 
therefore not useable by kronolith.
I figure, to fix this I'd have to add an entry to the "map" parameter, 
like the "localsql" does it, like this:

   $grpSource = array(
       'title' => _("Group"),
       'type' => 'group',
       'params' => array(
          'name' => ''
       ),
       'map' => array(
           '__key' => 'email',
           'name' => 'name',
           'email' => 'email',
           'freebusyUrl' => 'object_freebusyurl'
       ),
   // [...]

This does add the "Free/ Busy URL" entry to the show contact thingy, 
however, I have no idea where to fill in the value so it shows 
http://<myhost>/horde/kronolith/fb.php?u=<userid>.
Simply adding

   $cfgSources['group_' . $_group_id]['freebusyUrl'] = 
'http://localhost/horde/kronolith/fb.php?u=' . Auth::getAuth();

and similair to the loop below does exactly nothing :/
So I guess my question is: Does anyone know where I'd have to add that code?
Or is there a more direct solution for my problem that I have missed?

Johannes