How could one think about DDoS'ing something relaying on thousands of segregated nodes? I don't see any point in this discussion.
I am not suggesting that any DDOS attackers will actually think they can bring down the network compeltley. But they will be able to sow confusion, and use that as a "marketing tool". It all part of a potential FUD campaign.... sow fear as in "Load a Classic Node and you'll be attacked". Happens all the time. Just because they can't win doesn't stop them from throwing an epic rage attack. Sort of like Mike Hearn throwing an epic rage quit hissy fit, blockstream/core supporters plan on going out in equally destructive style it appears.
I think the real danger will be in the possibility of a "Code War". Imagine this scenario..... At the same time, or shortly after, Classic is released, Core uploads some patches designed to throw a monkey wrench into Classic. Then, after causing hte problem, they'll launch huge PR campaigns about the dangers of everything going on. And mass DDOS attacks will take out highly visible infrastructure players.
IT WON'T MATTER that all of this is temporary. All that will matter is that it gets public attention, and that fear and doubt spreads. Remember, all Core/Blockstream supporters have to do is fight off the 75%. But if that fails - then AFTER the 75% triggers Classic, there will be a month in which everyone is allowed to prepare for Classic, upgrade their node etc. But what if Core/Blockstream supporters take that time launch bad code, FUD Campaigns etc.
What happens when 2 Chains emerge, and Core/Blockstream supporters decide to fight to the death for their chain? What if there is competing Bitcoin News Media streaming out different versions of "truth" about what exchanges to use. An uninformed bitcoin user could easily get locked into the WRONG chain, and see their bitcoins go to zero eventually, or perhaps just die slowly on the eventual Alt-Bcoin chain.
So yes, DDOS will most likely play a role in what I see unfolding. Won't be the whole role.... but all good wars have multiple battle strategies. And this is going to be a minor war. Bigger war will be that taking advantage of all this confusion, Fed Coin will launch.