Newsnight ran on a Saturday for 5 or 6 weeks during the 2010 General election iirc.
Maybe I'm remembering incorrectly but I think they did one during the phone hacking saga too.
Huw too has certainly done Sundays in recent times when the news has justified it.
I do wonder too if they'll draft in one of the 'big guns' for Breakfast on Sunday to cover the fallout of the vote rather than the usual lucky dip they do normally to pick Sunday presenters.
That said I'd be VERY surprised if Dan Walker were to appear!