happen to have a link? I’ve had a good look around and not found any great info. Though I did see some old posts by galaxyman who seemed to get a great result, but he did not give details.
Thats a great sounding suggestion - and perhaps a better option, but before diving that deep I’m trying out altering scenecapture2d to accept a 2d rect bound so I can minimise render time (though, memory usage would still be fairly extreme, particularly with recursion)
It looks like it should not be so hard - but it seems this is not the reality of the situation 
That sounds like an excellent solution for allowing portals to be placed against walls - somewhat disappointed in myself I did not think of it.
Though it does not solve the issue of collisions with a clone at the other end of the portal, I think this is dooable with some low-ish level hooks in physx.