Did you ever do any poking around with late binding, to see if it's possible to get one DLL that will work in multiple versions? That's something I'm planning on looking at once I have 2008.
At the moment, it might not be too much of an issue. C3D is changing so fast that I can't see staying on ANY version once a new one is out, unlike what people do with the other versions. That's the main reason why I suspect it won't be a big issue if they don't release a patch for the Curve class API, because it seems like an actively bad idea to stay on old versions of Civil-3D.