Hi there,
A buddy and I had a post-Thanksgiving backcountry stay on Maple Leaf Lake (Saturday) several years back. We did get to the park fairly early (around 9), and had no problem with our pick of sites. Having said that, it wasn't the best weather (grey with a couple showers), so it may be a bit different if the weather is nicer. Provided you get to the park early, I'd be surprised if there wasn't something available at Maple Leaf or Guskewau for a shorter hike. Failing this, I'd be totally shocked if all sites within 12 km's of the hwy 60 trailhead would be booked up.
On a side note, I've also noticed that backcountry sites are booking up considerably more this year. I'm not sure if it's because backpacking is getting popular, if it's a reaction to car-camping sites being unavailable, or the awesome weather getting people out - maybe a bit of all these things? I for one would really love to see more backcountry hiking/camping options in the near north to accommodate this!
Hope this helps,
Rich