I will try to explain how to attach a camera to a portal door in Quake3.
First of all lets create a brush that will be used for our portal. Texture it with sfx/portal_sfx.
Now add a misc/misc_portal_surface entity and place it in front of our portal brush (entity can touch the brush).
Add another entity - misc/misc_portal_camera and place it in front of the area you want to show in our portal reflection. Now first select the misc_portal_surface, then misc_portal_camera and connect them with Ctrl+k.
Unfortunately thats not all. You need to place one more entity called target_position near the misc_portal_camera (target_position should be aiming at the area you want to display in our portal).
Select the misc_portal_camera then target_position and connect them with Ctrl+k.
Thats it! If you followed all the steps carefully, you should be able to see a nice portal with a camera attached to it. If you add the teleport trigger in front of our portal, you will have a working example of dm7-like effect in your map.
Note: If you see an upside-down view in the reflection, just select the misc_portal_camera entity and type 180 in the 'roll' input field.
Note: It's not wise to set up a camera in detailed rooms because it can drastically decrease FPS speed. What you actually see in the portal reflection is not a texture or any static surface - it's real brushwork, so the number of polygons are multiplied by two.