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Strictly-speaking, any moderator can leave a message along with Helpful, @Bill - there's just no UI for it.
The following bookmarklet, when clicked after loading the mod flag dashboard, will give an additional option (with a textbox) "custom helpful" which lets the moderator mark a flag as helpful and leave a message (similar to a decline message) with the flag.
javascript:function manishCustomHelpfulappendPopup(postid){$('<div id=manishCustomHelpful><input id=manishCustomHelpfulText type=text style="width:90%"></input><br><input id=manishCustomHelpfulButton type=button value="Custom Helpful"></input></div>').insertBefore('input[type=button][value=helpful]');$('#manishCustomHelpfulButton').click(function(){$.post("/messages/delete-moderator-messages/"+postid+"/"+renderTimeTicks+"?valid=true",{fkey:StackExchange.options.user.fkey,comment:$('#manishCustomHelpfulText').val()});$('[title="close this popup (or hit Esc)"]').click();$('#flagged-'+postid).hide();});};$("table.flagged-posts.moderator .dismiss-all").click(function(){var postid =($(this).closest('tr[id^="flagged-"]').attr('id').replace("flagged-", ""));setTimeout(function(){manishCustomHelpfulappendPopup(postid)},200);});
