I have used the following script to send all comments from our Main Jira project to our Service Manager project as internal for the last 11 months, but we recently made a decision to send the reporter through as Exalate rather than as the customer. Comments are coming over fine, but are not coming into Service Manager as internal comments. 

The script is:

if (executionInstanceName == "ServiceDesk") 
issue.comments  = commentHelper.mergeComments(
           issue, replica, { it.internal = true; it })

issue.comments  = commentHelper.mergeComments(
           issue, replica, {            
               comment ->
                 comment.body =  "|https://wenergysoftware.atlassian.net/browse/"+replica.key+", "+ comment.author.displayName +  " commented: \n" +comment.body
   }
)