Looking for links to projects where video decoders are used as general purpose ADCs
I like this idea.  You may find that the level clamping and line buffer xcircuitry may prevent you from using it as a general purpose ADC device.

Check as many data sheets as you can find, some chip might have a debug, or level aquisition mode that streams raw samples.

I would see if there is any information relating to the opensource Software Defined Radios that makes use of commodity interfaces.  Something in this line may be able to serve your purposes.  Also there are some DIY digital scopes that may serve but this may be exactly what you are trying to replace.