Ringing on my OSD project
If you're just trying to overpower a video signal without gating it, your effective source impedance is going to vary every time your video is switched on and off.  Not good.  You need to ensure that both sides of your video connection see a uniform 75-ohm impedance whether you are passing through the signal or supplying your own.  The easiest way to do that would be to use a pair of video amplifiers and put your circuit in the middle.  The input amplifier should probably have 1:1 gain and a low-impedance output; use something like a 4066 to switch that and a white signal.  The output amplifier should have a 2:1 gain and a 75 ohm resistor in series.  BTW, are you doing anything for DC restoration?  That could be an issue as well.