Short to Power Protection for ADV7180 and ADV7393

First of all direct AC coupling may not solve your problem.  Load dumps are like very slow positive ESD hits and depending on the cap size, the encoder will still see the rising edge coupled into it.

I'd buffer the video signal such as shown in the ADV7390 datasheet Figure 96.  This puts a 75 ohm resistor and buffer between the 100V and chip.  After this you'd have to put an active load dump protection in series with the video signal.  Fortunately load dumps are positive so you can use a NFET as your inline switch with detection circuitry.  You can check out TI app. note snva190a as a basis but you'd have to change things a bit.  Be careful of capacitive load on the video signal, too much and lower the bandwidth width on the 75 Ohm cable.  Also make sure the inline switch you select can handle the bandwidth.

I'd follow the same approach for the input side,   active switch -> 75 Ohm resistor -> 2xbuffer -> ADV7180.

On another note the fault you are protecting against is video cable insulation break down in the cable bundle.  Is it possible to double insulate the video lines.  Maybe a wrapper around all the video wires.  Then you'd have to have a double fault to damage the video chips and in general we don't have to protect against double faults.  As a matter of fact the power cables in the bundle already have insulation and the video cables have insulation so you have double insulation between wires where both insulations must break at the same point for the 100V to get on the video lines.  I'd still buffer the video signals.  Just a thought. 

Personally I've never had to protect video signals from load dumps but I have used this approach in other applications.  Maybe somebody else has a better solution.