Using The Real Thing instead of skyboxes

I'm curious: is anyone who is currently working on Eternity maps trying to use a roof layer that covers the entire level, and the mountains seen in the distance happen to be actual sectors belonging to that layer? And the mountains should not be seen thru a skybox camera, but thru a (linked) portal that connects the gameplay area to said roof layer. Going further, the clouds could be actual translucent linedefs drawn way above.

If someone did try this, was it resource intensive?

I'm using portals that usually cover the entire playable area and some of its surroundings, topped off with a skybox. The skybox is just a textured cylinder with no top; this due to the fact that I didn't like the angle artifacts that the usual square box seemed to show... and my inability to match the top with the sides :)

Trying to do a skybox without actually using a skybox will not look good. You'll notice the dimensions of your skybox when moving around, due to the angles of the sidedefs relative to the player.

I've experimented with doing stuff (eg animated clouds) in the skybox itself, but haven't come up with something satisfying. Part of the problem is that the smallest possible movement speed still goes pretty fast.... it might work for a stormy kind of sky.

At least one level (so far) has a vantage point that allows you to see the entire level from above. I do notice that my 6+ year old machine awakens from its slumber for a sec when I go up that vantage point, but nothing that distracts from gameplay.

Probably it would be outrageous to extend the roof layer to miles breadth and also draw sector mountains that are miles broad.

I'm "cheating" that by having the playable area rimmed with mountains/cliffs. But if you just sketch far-away mountains you can still get away with that. Slopes are amazing for doing this.