It would still be required for the content producer (ie, the content-spam-farm) to label their content as such. The current approach is that the content served

It would still be required for the content producer (ie, the content-spam-farm) to label their content as such. The current approach is that the content served is the same for humans and agents (ie, a site serves consistent content regardless of the client), so who a specific header is "meant for" is a moot point here.