Not really a hack. Subcarriers are how they added color to TV, stereo to FM radio, and so on. WWV/WWVH also sends machine-readable time codes at 100Hz like this.
Set your time machine for 1958, or follow this link [0] to see how RCA did both FM stereo (throughout the article they call it "multiplex") and 67 KHz "SCA" subcarriers. Admire the awesome 5KW FM transmitter cabinetry and transformer shielding.
According to Wikipedia, the transition started in 1960, so we're two years away when this article was published.