As far as I can tell, there is no way within civicrm core to enable SSL connections to a remote database. This would be a major concern for anyone running CiviCRM with separate web and database servers. I've 'solved' this issue for myself by making the changes to civicrm/civicrm/packages/DB/common.php and civicrm/civicrm/packages/DB/mysqli.php detailed here, but it's rather sloppy and may need to be re-done after an update. Would be very useful to be able to set a flag within civicrm.settings.php (or even the initial install page) to enable SSL.