the Portal BSDF was merged into the main branch of blender yesterday, so its in the latest daily build and you can play around with it
its a bit of a doozy to set up, I used the setup from the blend files provided in the pull request linked above and modified it a lil to also take portal scale into account, so i can finally make one of those classic "size portal" pics
or well, the ray portals will only teleport rays, so if you want to shove actual geometry through the portals you'll have to set it up yourself with geometry nodes, but that's a task for another day (i am the very tired today)
