I should have been more careful with my words. My enthusiasm got the better of me: I don't think an employer will be bold enough to claim that some agent a person brings to work is the employer's property. If I download a script I find on Github and use it at work, nobody is going to argue that gives my employer the ability seize ownership from whoever made the script available.

Yes! This is exactly where I'd like to see things go. It makes no sense to claim ownership of an idea. It might make sense to claim credit for it. But once the knowledge has passed to one other person, you lose all control over it. It's just how knowledge works. I think most efforts to control the flow of information are probably going to do more damage than they avert.