Adding functionality to limit results to a specific domain

I recently had a client request that we develop a way to limit results in searches to only the current domain in a multi-domain setup using Domain Access. My fix was to leverage this module with only a bit of extra code in an additional custom module:

/**
 * Implements hook_apachesolr_prepare_query.
 */
function mymodule_apachesolr_prepare_query(&$query, &$params, $caller) {
  if (module_exists('domain_solr') && $caller == 'apachesolr_search') {
    global $_domain;
    $query->add_filter('im_domain_id', $_domain['domain_id']);
  }
}

I'm a bit torn as to whether this would be best as its own separate module, or as a feature of this one. Since the main purpose of this module is to make the facet block, and this code actually implies that you don't want the block enabled (you just want results filtered automatically), I'm not sure which path to go.

What do you think? It would be as easy as replacing the module_exists() call with a variable_get() that's set somewhere in the admin to say "Do you want all search results limited to just the current domain?"