How to use Solr Handlers when sending requests from Drupal
Ok, we figured out how to do this. In order to choose the handler that is being used by Solr while sending a request from Drupal, we have to edit hook_apachesolr_query_alter() and add the following code:

$query-&gt;addParam('qt', 'MyHandlerName');


We did some extra coding to allow us to change the handler directly from back office in order to be able to switch handlers without touching the code.