Google has already indirectly "extended" Native Client with native 3D graphics using their O3D API (http://code.google.com/apis/o3d/). It's just a matter of time until someone codes up a library to expose that API through JS interop in Native Client.
I think we can expect very close integration of Native Client into the Chrome OS as soon as or right after it goes public. To the level of accessing hardware ports to control external devices right from your Native Client web-app.
Imagine drivers that get loaded and updated on-the-fly. Of course, Google will have to implement a smart security mechanism (along the lines of RSA authentication for dynamic device/driver coupling) and provide a way for device manufacturers to register their driver's public keys with Chrome OS.