I've had a quick look, and from what I can tell, you're right... but I have a slight concern that they might not like (a) the load (when it cranks up to full speed next time there's a hurricane loose in the gulf) or (b) the fact that users technically don't have to 'view' the app at the Ning site. Once they have the KMZ file, it's all viewed within GE, and Ning is just 'pulled-from'. I'm wondering if (longterm) they'd prefer vistors to the site in a browser, because that way they get to see the ad-banners which (may) ultimately generate the revenue for the Ning project to survive.
Still, I'm not saying it WON'T work, at this point...
I'll keep an eye on Ning, and see what develops. If I can get a login, I might find out a bit more about any restrictions.