Certainly making it a display mode - similar to the technical modes - would be easiest solution to make sense of. It might not be quite as powerful as having different toon materials, but it would be quick and easy to set up.
And SetObjectDisplayMode would work for adding individual objects in toon style.
Is NPR now implemented in V6? I saw something related in this link
If it is ready to use, some explanation about how to use this feature would be appreciated.