Well, you could achieve this, but it would take more than just LSL, you would essentially need a customised version of the viewer and some bot accounts. The bots would be your cameras, either streaming or generating snapshot type images.
Make sure the output is accessible via the web and use media on a prim to display it back in world.
Lots of work and lots of your own bandwidth would be needed.
I get what you are saying about a 'nice to have' feature, I've thought the same thing myself about having true reflective surfaces, but features like these have the potential to utterly destroy performance, particularly if overused or deliberately misused. Reflections are bad enough and they would only need to be rendered on the client side.
But a camera/screen setup, even if rendered by the client, would place a lot of extra load on the servers. The server would have to send extra data, for areas you normally could not see. Even if its restricted to only working in one sim you would either effectively have to ignore view distance and send all data for everything in sim, or determine which screens were in view, which cameras they were related to and what was in view of those camera(s) and send that in addition to the usual set of data. Remember that although a sim is only 256m per side, it is thousands of meters high.
Multiply that extra data by the number of sims, number of cameras and number of avatars present and consider the impact of lots of cameras/screens on bandwidth used by the server.