Find a suite of third party applications and drivers that depend on the undocumented APIs otherwise your not going to get very far with getting your code in.
Why is that? There are varying amounts of "undocumented." Not documented on MSDN doesn't mean that no documentation exists. For instance, there's Gary Nebbett's "Windows NT/2000 Native API Reference." Personally, I've thought that playing around with the native LPC API might be interesting. I'm sure there are other areas of the native API that are sparsely documented, and for which some test cases might prove useful.
--Juan