New configuration variable $TYPO3_CONF_VARS[‘FE’][‘additionalAbsRefPrefixDirectories’] can be set to an array of additional directories to which config.absRefPrefix will apply. If not set, it will only apply to media/ typo3conf/ext and fileadmin/ directories by default.
More info:
RFC #9452: config.basRefPrefix is applied only to a fixed set of directories
http://bugs.typo3.org/view.php?id=9452