It's an interesting idea, but we had a thread a while back for suggestions to prevent multi's and I think the reason most of them were declined was because they don't want to put up restrictions for potential new members, so I'd imagine this would be looked at with the same reasoning.
It's here if you'd like to take a look (it's closed now), I don't think anyone had suggested what you're saying -
http://www.thisisbigbrother.com/foru...d.php?t=297949