I’ve got a (mostly) working version of this going with SceneCaptures, what’s trickiest about it isn’t the rendering, it’s collision information. An object can only occupy one space so if half of it should exist across the portal, it becomes incorrectly defined. For showing this, it’s not a big deal (you can use some depth fade tricks to keep objects from clipping into the portal) but trying to get traces or overlaps to register is a nightmare, and a lot of particle effects can’t be transformed by the portal itself when certain particles cross the boundary (animtrails are a glaring example of where the world coordinate disconnect becomes a real issue).
So rendering portals, that’s not the killer element here; what I’m really interested in is seeing how alternate solutions handle collision of objects across the portal plane