Custom Mailing Labels and "Merge labels for contacts with the same address"
The answer is already here. According to a tiny mention at the bottom of http://wiki.civicrm.org/confluence/display/CRMDOC/Directories page, it is possible (and, more important, it works!) to copy modified core files to Custom PHP Path Directory (at civicrm/admin/setting/path?reset=1) , preserving original files/folders structure, so next time CiviCRM gets security updates, changes should be kept (unless original file / function major changes).