Does Nexuiz have render to texture? If so, is there a way to make cameras and monitors in a map?

The Darkplaces engine does not support render to texture.

However, you could use a piece of pre-recorded video as an animated texture if that interests you:
http://forums.alientrap.local/viewtopic.php?t=3715

Ed wrote:The Darkplaces engine does not support render to texture.

OT but, how difficault would implementing render-to-texture be?

Very difficult for the server side, probably easy on client side. The problem is that the server currently would not send any entities not in view, so you'd just see the map through the screen, but not the players walking along.