It has been said in the comments and I think it bears repeating in an answer that NaNo has its own set of forums and Q/A. There is a lot of internal support for NaNoWriters on their site. I don't think Writers can compete in that area, but I do think NaNo is a good event to leverage for increasing our traffic.
What Writers.SE can offer is a place to chat, and we might consider supporting something like Come Write In events, similar to the way a local library works. If we advertise this across the SE community via community ads or other means, we can pull traffic from other sites and potentially increase the size or at least visibility of our community.
In the past two years, our Writers chat group (Tuesday afternoons Eastern time-ish) has had participants in NaNo that have used the Writers chat room to meet, discuss our challenges, offer support, and do writing sprints. We did some small grassroots efforts to bring in users from other SE sites that seemed like they would be most relevant. These were: Sci Fi & Fantasy, Role-Playing Games, Arqade, Game Development, and English Language and Usage. We were successful at recruiting a handful of interested users from other sites, some of whom stuck around after NaNo.
I think we should do it again this year, and try to expand our reach to cover more SE sites. We are more experienced this year than ever before, and I am more confident than I have ever been that we can create a helpful workspace for NaNoWriters. I think we should focus on SE users because we have lots of room to grow, and SE users will already be familiar with the site/chat mechanics.
As for topic challenges, remember that you are thinking of squeezing some more words out of NaNoWriters, who are training to ignore their inner critic and just write. I'm not sure these two things will go together. On the other hand, I think we have a good body of knowledge about how to prepare for NaNo and how to write a draft of a novel in a month. Are questions like that, which are really pretty specific to NaNo due to the artificial time constraint, general enough to be on-topic? If so, I can probably brainstorm at least a few of my basic NaNo prep/pep talk points into actually questions and answers.