How to: Do that TV thing.
The way Half-Life TV's and screens work, is that they have an area somewhere in the map, that can only be reached via noclip. In that area the thing that is supposed to show up on the TV/Screen/whatever is acted out by an NPC or whatever. An invisible camera then projects that to the TV/Screen/Thing.
This means that if you find this area and remove the NPC that's already there (or any other thing that is taking up the space) you can place your own ragdolls or props, and have them show up on the TV/Screen/Stuff.