for 1.6 RC5
The following should work based on the code of osTicket v1.6 RC5. In include/class.ticket.php around line 617 you should find
$body=$this->replaceTemplateVars($body);
$subj=$this->replaceTemplateVars($subj);
$body = str_replace('%signature',($dept && $dept->isPublic())?$dept->getSignature():'',$body);
After these lines add
$body = str_replace("%message", $msg,$body);