very much depends on which camping place you get a ticket for, if its Maision Blanc or Houx you can forget finding space that late. But if its one of the newer sites you might find a spot.
Sorry to disagree with you Nick....
If you get a Houx pass you will get an allocated spot - if you're arriving late this is your best option for finding space.
MB this year is in theory in marked out plots so one would hope that there will still be space left on Saturday for those who hold passes, although this year will be very much "suck it and see".
On the sites without marked pitches (BSJ, HA, Expo, Technopark (?)) you should just be able to walk onto the sites and pitch even though you don't have a pass. You would only need a pass for a car. If you've only got a small tent I'd imagine you'll find space on the saturday in most of the sites as long as you don't mind being squished a bit in a corner!