The best you could do since there is a limitation with the Proxy user on Github, the name cannot be modified. Then, you can impersonate the author on comments as you add an indicator string. So yo could do:
issue.comments = commentHelper.mergeComments(issue, replica,
{
comment ->
comment.body =
"[" + comment.author.email + "] " +
comment.body + "\n"
}
)
//This will help to add a reference from the email/author coming from Jira on each GitHub comment.