Is there a way to force some block caching despite system block caching option is disabled?
With latest Drupal 7.33 release there is a new setting variable now:

/**
 * Block caching:
 *
 * Block caching may not be compatible with node access modules depending on
 * how the original block cache policy is defined by the module that provides
 * the block. By default, Drupal therefore disables block caching when one or
 * more modules implement hook_node_grants(). If you consider block caching to
 * be safe on your site and want to bypass this restriction, uncomment the line
 * below.
 */

$conf['block_cache_bypass_node_grants'] = TRUE;


Setting the above to TRUE the "cache blocks" option becomes checkable.