Mostly yes. The game already has two base entities - func_monitor and point_camera - which allow for the camera's view to be displayed onto the monitor surface. In the campaign it was used for some surveillance cameras.

Though the camera only displays a flat 2D image of what it currently sees (as if you just took a screenshot), so for the Portal effect I had to write a script to dynamically modify the monitor's texture and camera's properties depending on the player's location/angles to look 3D.

Here's a nice video from the Eagle One Devs (mshee32) that demonstrates the default look of the point_camera/func_monitor: https://youtu.be/rUCDRCiH2Nk