That's just a title tooltip isn't it? You can set those in Markdown easily enough (eg. [display](http://hyperlink "hover text")), and you're not allowed any sort of markup inside the tooltip, and they have severe length limitations too. So it'd be a major compromise. (I have, painfully, added them to the frontpage of gwern.net, but no one has ever commented on them or given any sign that they are useful, so I've never bothered with putting them elsewhere.)