I was definitely looking at those seamless portals videos earlier.
Actually the more modern source ports of DooM like Zdoom have straight up fancy projected skies, just like source engine.
I was starting to think about rendering to a render target of some sort with a camera set in a pocket world that matches the looking direction of the player and seeing if my material can sample that texture. I could also make that camera move slightly in actual space to give the sky a tiny bit of parallax as you move. Then I don’t really even need a cube map and worry about rendering quality of the sky. It might tank perf if the sky is super complicated with geometry that won’t even be visible most of the time, but I think I’m OK taking the hit, especially if we keep the sky fairly simple for this. Even in Source games if you go to the sky world, the geometry is super low detailed in there.
It would especially be useful if the render target can avoid being even rendered to unless the sky is visible.