PMB, as usual you're right. It was more meant as a rethorical question, concering all the hacking that's going on, bypassing all the nice interfaces Valve was making up for us
In that case, it works by just including/adding networkvar.h/.cpp to the project.
However, it could have been that new dependencies arise, and the whole game would have continued until you finally would have to include the whole SDK to satisfy all dependencies.