JHILLA, diplexers work with satellite receiver signals because the DC is WAY far away from the RF frequencies involved, so it is easy to build a small filter to separate and combine the DC and RF. Diplexers are usually rated on the antenna/cable side from about 50 - 850 mHz and on the satellite side from 950 - 1450 MEGAhertz. Composite video starts with the 60 HERTZ sync pulse, and only goes up to 6 mHz with the fine picture detail and the color signal (if present).

I see my earlier reply includes a nonsense sentence that I did not catch before I sent it: <>

This should be "I don't think you could make a power inserter because the sync pulse of the video signal pulls the video voltage from 0.3 volts to 0.0 volts about sixty times a second."

And, sure enough, the sync pulse will never be able to pull the video signal from 0.3 volts to 0.0 volts if you impress 12 or 24 volts DC on top of the video signal. Without sync, there is no image.