I think the autocomment feature when voting to close as duplicate is fantastic. It helps the person who asked the question and alerts others to relevant questions.
It seems to me that a similar feature when voting to close as off topic would be equally helpful. Sometimes users voting to close don't take the time to alert the user that s/he may get better answers to the question on a different site. It's quite likely that such automatic comments would prompt the user to delete the question and repost it the other venue, thereby saving time and effort of moderators.
I'm thinking, just a little comment that reads "This question may be better suited for ...." If none of the 5 SE options is selected, then either no comment or a comment that simply reads "This question may be off topic for SO."