As this is a button beneath the edit box it would have to be fully, properly written into the PHP that is MediaWiki - i.e. this function would
This can easily be done in JavaScript and thus be enabled by every user on an opt-in basis. If there is serious interest in this I could whip something up. With a bit of extra work/AJAX this could even be extended to SubSection saving and restoring only.
But the problem I see with this feature is article forking (as others have said). Someone wants to overhaul a page and saves it as a draft, in the mean time other users edit the original article and... desaster :) Maybe only activate the feature for new articles by default?