So. I was messing out with HL1 vanilla, what can i do there. And Found how you can not only make 3d skybox in it, you can tie it to specific room where you currently in allowing hotswap of skyboxes. That is proof of concept (skybox used for far sky, is from Enriched). And techinque itself similar to how Enriched to it, but meshes here exceed vanilla HL1 grid FAR FAR beyond. ANd i kinda proud that i figure out how to do it. Bare minimum what you can do by this techique - 512x skyboxes, and if you don't want to - just 3d models that would be very big and far away. Also, Env_light here working too.
(Edited)
See cry of fears skyboxes, they use 3d skyboxes with a similar system to source
As spirit of half-life. I know. But intention as make it in vanilla, so you can launch it. Idk, in opfor, or BHL or MMoD