Greets. :)

Would releasing the art itself as CC-BY-SA and warning people that the actual characters are CC-BY-NC-SA, and that they would still have to ask permission for commercial use be legal and okay?

Sorry, no.

If not, could you please point me to a site that allows CC-BY-NC-SA art to be submitted?

The first site that comes to mind is deviantART.  While it's not specifically for video games, they have a wider selection of CC licenses available.  If you want to draw attention to them, I recommend announcing them on the gameassets subreddit.  You're also welcome to mention them (and link to them) in the descriptions of your submissions here.

Another option would be to just go with CC-BY-SA. Anyone with commercial interests won't touch your content with a ten-foot pole; I speak from experience. The NC clause only stifles the little guy who might want to promote your work on a blog that happens to run ads (and makes no money from them, actually). Like mine, for example. And yes, the NC clause really is that broad. Maybe a court of law would decide otherwise... but I can't afford to take such a chance.

"Another option would be to just go with CC-BY-SA. Anyone with commercial interests won't touch your content with a ten-foot pole;"

I don't think that's a reliable way to prevent commercial projects from using your assets.

Edit: nevermind. I was thinking of CC-BY and separate asset collections of CC-BY-SA.

I don't think that's a reliable way to prevent commercial projects from using your assets.

Not if you want legal guarantees, no. But I assure you that most commercial projects will stay as far away from CC-BY-SA content as they can. I did mention speaking from experience, right? It happened when friends of mine with a commercial gaming portal asked permission to translate some of my blog posts. I pointed out the CC-BY-SA mark at the bottom, and suddenly they lost all interest -- even when I offered them an exclusive license for the translations. Similarly, you know Getty has a partnership with Flickr to license photos for commercial use? They won't touch anything with a CC license either. The one time a (different) company did approach me to license a photo for a commercial product, they asked for explicit permission, even though they didn't need to! Even a student did this, at the guidance of his teacher. And it was for a strictly non-commercial school project!

So yeah, I'm pretty confident about this. Please, please stop scaring honest people away from your content. That's not what protects you from abuse.

"Would releasing the art itself as CC-BY-SA and warning people that the actual characters are CC-BY-NC-SA, and that they would still have to ask permission for commercial use be legal and okay?"

But in that case, what use would having the art itself as CC-BY-SA, if people are still prevented from using it commercially? Or do you mean, you are okay with people using those specific pieces of art commercially, but not creating new pieces of art with your characters? This is potentially a bit confusing, since CC-BY-SA allows people to create derivative works, including commercially.

Note that non-commercial licences prevent people from using the work both in commercial games (obviously) but also Open Source games. They can still be used in "freeware" games where developers release for free but don't care about everything being under "Free" licences. Though as noted, there are all sorts of issues, such as whether being distributed on a website with ads, or on a magazine cover DVD.

Commercial use isn't always pure evil. If you believe non-free licenses are unethical, then go with a copyleft license, which is what CC BY-SA is. This way, people can't release derivated works under a more restrictive one.

Creative Commons BY-NC-SA is not a free license and I wouldn't advise anyone to use it.

See http://freedomdefined.org/Licenses/NC for more information.