Unable to override ctools selection plugin class from my module
same problem, two days wasted according to http://lookalive.co.uk/blog/drupal-entity-reference-how-make-custom-entity-reference-selection-handler and all it's copies.

entityreference_get_selection_handler() code dpm() showed that "*.inc" should be named 'base.inc' as in your case.

After renaming in my case *class.php started to execute.

So all i can advice to you - check if module has higher weight than 'entityreference' module, see here https://www.drupal.org/node/110238

And add the first line to '*class.php' (for inheritance)

require_once drupal_get_path('module', 'entityreference') . '/plugins/selection/EntityReference_SelectionHandler_Generic.class.php';


If this helped to you - pleace vote [+] this answer (i need more scores, tks)